Make Meter a struct, simplify the global Meter (#709)

* Tests pass

* Precommit pass

* More lint

* Remove a few interfaces

* Final edits

* Fix comments

* Indentation

* registry->unique

* Comments

api/global/global_test.go

@@ -37,7 +37,7 @@ func (*testTraceProvider) Tracer(_ string) trace.Tracer {
 }
 
 func (*testMeterProvider) Meter(_ string) metric.Meter {
-	return &metric.NoopMeter{}
+	return metric.Meter{}
 }
 
 func TestMultipleGlobalTracerProvider(t *testing.T) {

api/global/internal/meter.go

@@ -49,22 +49,27 @@ import (
 type meterProvider struct {
 	delegate metric.Provider
 
+	// lock protects `delegate` and `meters`.
 	lock sync.Mutex
-	meters map[string]*meter
+
+	// meters maintains a unique entry for every named Meter
+	// that has been registered through the global instance.
+	meters map[string]*meterEntry
 }
 
-type meter struct {
+type meterImpl struct {
-	delegate unsafe.Pointer // (*metric.Meter)
+	delegate unsafe.Pointer // (*metric.MeterImpl)
-
-	provider *meterProvider
-	name     string
 
 	lock       sync.Mutex
-	registry   map[string]metric.InstrumentImpl
 	syncInsts  []*syncImpl
 	asyncInsts []*asyncImpl
 }
 
+type meterEntry struct {
+	unique metric.MeterImpl
+	impl   meterImpl
+}
+
 type instrument struct {
 	descriptor metric.Descriptor
 }
@@ -73,8 +78,6 @@ type syncImpl struct {
 	delegate unsafe.Pointer // (*metric.SyncImpl)
 
 	instrument
-
-	constructor func(metric.Meter) (metric.SyncImpl, error)
 }
 
 type asyncImpl struct {
@@ -82,7 +85,7 @@ type asyncImpl struct {
 
 	instrument
 
-	constructor func(metric.Meter) (metric.AsyncImpl, error)
+	callback func(func(metric.Number, []core.KeyValue))
 }
 
 // SyncImpler is implemented by all of the sync metric
@@ -107,7 +110,7 @@ type syncHandle struct {
 }
 
 var _ metric.Provider = &meterProvider{}
-var _ metric.Meter = &meter{}
+var _ metric.MeterImpl = &meterImpl{}
 var _ metric.InstrumentImpl = &syncImpl{}
 var _ metric.BoundSyncImpl = &syncHandle{}
 var _ metric.AsyncImpl = &asyncImpl{}
@@ -120,7 +123,7 @@ func (inst *instrument) Descriptor() metric.Descriptor {
 
 func newMeterProvider() *meterProvider {
 	return &meterProvider{
-		meters: map[string]*meter{},
+		meters: map[string]*meterEntry{},
 	}
 }
 
@@ -129,8 +132,8 @@ func (p *meterProvider) setDelegate(provider metric.Provider) {
 	defer p.lock.Unlock()
 
 	p.delegate = provider
-	for _, m := range p.meters {
+	for name, entry := range p.meters {
-		m.setDelegate(provider)
+		entry.impl.setDelegate(name, provider)
 	}
 	p.meters = nil
 }
@@ -143,29 +146,24 @@ func (p *meterProvider) Meter(name string) metric.Meter {
 		return p.delegate.Meter(name)
 	}
 
-	if exm, ok := p.meters[name]; ok {
+	entry, ok := p.meters[name]
-		return exm
+	if !ok {
-	}
+		entry = &meterEntry{}
+		entry.unique = registry.NewUniqueInstrumentMeterImpl(&entry.impl)
+		p.meters[name] = entry
 
-	m := &meter{
-		provider:   p,
-		name:       name,
-		registry:   map[string]metric.InstrumentImpl{},
-		syncInsts:  []*syncImpl{},
-		asyncInsts: []*asyncImpl{},
 	}
-	p.meters[name] = m
+	return metric.WrapMeterImpl(entry.unique, name)
-	return m
 }
 
 // Meter interface and delegation
 
-func (m *meter) setDelegate(provider metric.Provider) {
+func (m *meterImpl) setDelegate(name string, provider metric.Provider) {
 	m.lock.Lock()
 	defer m.lock.Unlock()
 
-	d := new(metric.Meter)
+	d := new(metric.MeterImpl)
-	*d = provider.Meter(m.name)
+	*d = provider.Meter(name).MeterImpl()
 	m.delegate = unsafe.Pointer(d)
 
 	for _, inst := range m.syncInsts {
@@ -178,49 +176,30 @@ func (m *meter) setDelegate(provider metric.Provider) {
 	m.asyncInsts = nil
 }
 
-func (m *meter) newSync(desc metric.Descriptor, constructor func(metric.Meter) (metric.SyncImpl, error)) (metric.SyncImpl, error) {
+func (m *meterImpl) NewSyncInstrument(desc metric.Descriptor) (metric.SyncImpl, error) {
 	m.lock.Lock()
 	defer m.lock.Unlock()
 
-	if meterPtr := (*metric.Meter)(atomic.LoadPointer(&m.delegate)); meterPtr != nil {
+	if meterPtr := (*metric.MeterImpl)(atomic.LoadPointer(&m.delegate)); meterPtr != nil {
-		return constructor(*meterPtr)
+		return (*meterPtr).NewSyncInstrument(desc)
-	}
-
-	if ex, ok := m.registry[desc.Name()]; ok {
-		if !registry.Compatible(desc, ex.Descriptor()) {
-			return nil, registry.NewMetricKindMismatchError(ex.Descriptor())
-		}
-		return ex.(metric.SyncImpl), nil
 	}
 
 	inst := &syncImpl{
 		instrument: instrument{
 			descriptor: desc,
 		},
-		constructor: constructor,
 	}
 	m.syncInsts = append(m.syncInsts, inst)
-	m.registry[desc.Name()] = inst
 	return inst, nil
 }
 
-func syncCheck(has SyncImpler, err error) (metric.SyncImpl, error) {
-	if has != nil {
-		return has.SyncImpl(), err
-	}
-	if err == nil {
-		err = metric.ErrSDKReturnedNilImpl
-	}
-	return nil, err
-}
-
 // Synchronous delegation
 
-func (inst *syncImpl) setDelegate(d metric.Meter) {
+func (inst *syncImpl) setDelegate(d metric.MeterImpl) {
 	implPtr := new(metric.SyncImpl)
 
 	var err error
-	*implPtr, err = inst.constructor(d)
+	*implPtr, err = d.NewSyncInstrument(inst.descriptor)
 
 	if err != nil {
 		// TODO: There is no standard way to deliver this error to the user.
@@ -264,29 +243,25 @@ func (bound *syncHandle) Unbind() {
 
 // Async delegation
 
-func (m *meter) newAsync(desc metric.Descriptor, constructor func(metric.Meter) (metric.AsyncImpl, error)) (metric.AsyncImpl, error) {
+func (m *meterImpl) NewAsyncInstrument(
+	desc metric.Descriptor,
+	callback func(func(metric.Number, []core.KeyValue)),
+) (metric.AsyncImpl, error) {
+
 	m.lock.Lock()
 	defer m.lock.Unlock()
 
-	if meterPtr := (*metric.Meter)(atomic.LoadPointer(&m.delegate)); meterPtr != nil {
+	if meterPtr := (*metric.MeterImpl)(atomic.LoadPointer(&m.delegate)); meterPtr != nil {
-		return constructor(*meterPtr)
+		return (*meterPtr).NewAsyncInstrument(desc, callback)
-	}
-
-	if ex, ok := m.registry[desc.Name()]; ok {
-		if !registry.Compatible(desc, ex.Descriptor()) {
-			return nil, registry.NewMetricKindMismatchError(ex.Descriptor())
-		}
-		return ex.(metric.AsyncImpl), nil
 	}
 
 	inst := &asyncImpl{
 		instrument: instrument{
 			descriptor: desc,
 		},
-		constructor: constructor,
+		callback: callback,
 	}
 	m.asyncInsts = append(m.asyncInsts, inst)
-	m.registry[desc.Name()] = inst
 	return inst, nil
 }
 
@@ -297,21 +272,11 @@ func (obs *asyncImpl) Implementation() interface{} {
 	return obs
 }
 
-func asyncCheck(has AsyncImpler, err error) (metric.AsyncImpl, error) {
+func (obs *asyncImpl) setDelegate(d metric.MeterImpl) {
-	if has != nil {
-		return has.AsyncImpl(), err
-	}
-	if err == nil {
-		err = metric.ErrSDKReturnedNilImpl
-	}
-	return nil, err
-}
-
-func (obs *asyncImpl) setDelegate(d metric.Meter) {
 	implPtr := new(metric.AsyncImpl)
 
 	var err error
-	*implPtr, err = obs.constructor(d)
+	*implPtr, err = d.NewAsyncInstrument(obs.descriptor, obs.callback)
 
 	if err != nil {
 		// TODO: There is no standard way to deliver this error to the user.
@@ -326,8 +291,8 @@ func (obs *asyncImpl) setDelegate(d metric.Meter) {
 
 // Metric updates
 
-func (m *meter) RecordBatch(ctx context.Context, labels []core.KeyValue, measurements ...metric.Measurement) {
+func (m *meterImpl) RecordBatch(ctx context.Context, labels []core.KeyValue, measurements ...metric.Measurement) {
-	if delegatePtr := (*metric.Meter)(atomic.LoadPointer(&m.delegate)); delegatePtr != nil {
+	if delegatePtr := (*metric.MeterImpl)(atomic.LoadPointer(&m.delegate)); delegatePtr != nil {
 		(*delegatePtr).RecordBatch(ctx, labels, measurements...)
 	}
 }
@@ -363,64 +328,10 @@ func (bound *syncHandle) RecordOne(ctx context.Context, number metric.Number) {
 	(*implPtr).RecordOne(ctx, number)
 }
 
-// Constructors
-
-func (m *meter) withName(opts []metric.Option) []metric.Option {
-	return append(opts, metric.WithLibraryName(m.name))
-}
-
-func (m *meter) NewInt64Counter(name string, opts ...metric.Option) (metric.Int64Counter, error) {
-	return metric.WrapInt64CounterInstrument(m.newSync(
-		metric.NewDescriptor(name, metric.CounterKind, metric.Int64NumberKind, m.withName(opts)...),
-		func(other metric.Meter) (metric.SyncImpl, error) {
-			return syncCheck(other.NewInt64Counter(name, opts...))
-		}))
-}
-
-func (m *meter) NewFloat64Counter(name string, opts ...metric.Option) (metric.Float64Counter, error) {
-	return metric.WrapFloat64CounterInstrument(m.newSync(
-		metric.NewDescriptor(name, metric.CounterKind, metric.Float64NumberKind, m.withName(opts)...),
-		func(other metric.Meter) (metric.SyncImpl, error) {
-			return syncCheck(other.NewFloat64Counter(name, opts...))
-		}))
-}
-
-func (m *meter) NewInt64Measure(name string, opts ...metric.Option) (metric.Int64Measure, error) {
-	return metric.WrapInt64MeasureInstrument(m.newSync(
-		metric.NewDescriptor(name, metric.MeasureKind, metric.Int64NumberKind, m.withName(opts)...),
-		func(other metric.Meter) (metric.SyncImpl, error) {
-			return syncCheck(other.NewInt64Measure(name, opts...))
-		}))
-}
-
-func (m *meter) NewFloat64Measure(name string, opts ...metric.Option) (metric.Float64Measure, error) {
-	return metric.WrapFloat64MeasureInstrument(m.newSync(
-		metric.NewDescriptor(name, metric.MeasureKind, metric.Float64NumberKind, m.withName(opts)...),
-		func(other metric.Meter) (metric.SyncImpl, error) {
-			return syncCheck(other.NewFloat64Measure(name, opts...))
-		}))
-}
-
-func (m *meter) RegisterInt64Observer(name string, callback metric.Int64ObserverCallback, opts ...metric.Option) (metric.Int64Observer, error) {
-	return metric.WrapInt64ObserverInstrument(m.newAsync(
-		metric.NewDescriptor(name, metric.ObserverKind, metric.Int64NumberKind, m.withName(opts)...),
-		func(other metric.Meter) (metric.AsyncImpl, error) {
-			return asyncCheck(other.RegisterInt64Observer(name, callback, opts...))
-		}))
-}
-
-func (m *meter) RegisterFloat64Observer(name string, callback metric.Float64ObserverCallback, opts ...metric.Option) (metric.Float64Observer, error) {
-	return metric.WrapFloat64ObserverInstrument(m.newAsync(
-		metric.NewDescriptor(name, metric.ObserverKind, metric.Float64NumberKind, m.withName(opts)...),
-		func(other metric.Meter) (metric.AsyncImpl, error) {
-			return asyncCheck(other.RegisterFloat64Observer(name, callback, opts...))
-		}))
-}
-
 func AtomicFieldOffsets() map[string]uintptr {
 	return map[string]uintptr{
 		"meterProvider.delegate": unsafe.Offsetof(meterProvider{}.delegate),
-		"meter.delegate":         unsafe.Offsetof(meter{}.delegate),
+		"meterImpl.delegate":     unsafe.Offsetof(meterImpl{}.delegate),
 		"syncImpl.delegate":      unsafe.Offsetof(syncImpl{}.delegate),
 		"asyncImpl.delegate":     unsafe.Offsetof(asyncImpl{}.delegate),
 		"syncHandle.delegate":    unsafe.Offsetof(syncHandle{}.delegate),

api/global/internal/meter_test.go

@@ -280,15 +280,15 @@ type meterProviderWithConstructorError struct {
 }
 
 type meterWithConstructorError struct {
-	metric.Meter
+	metric.MeterImpl
 }
 
 func (m *meterProviderWithConstructorError) Meter(name string) metric.Meter {
-	return &meterWithConstructorError{m.Provider.Meter(name)}
+	return metric.WrapMeterImpl(&meterWithConstructorError{m.Provider.Meter(name).MeterImpl()}, name)
 }
 
-func (m *meterWithConstructorError) NewInt64Counter(name string, opts ...metric.Option) (metric.Int64Counter, error) {
+func (m *meterWithConstructorError) NewSyncInstrument(_ metric.Descriptor) (metric.SyncImpl, error) {
-	return metric.Int64Counter{}, errors.New("constructor error")
+	return metric.NoopSync{}, errors.New("constructor error")
 }
 
 func TestErrorInDeferredConstructor(t *testing.T) {

api/metric/api.go

@@ -135,38 +135,99 @@ func (d Descriptor) LibraryName() string {
 	return d.config.LibraryName
 }
 
-// Meter is an interface to the metrics portion of the OpenTelemetry SDK.
+// Meter is the OpenTelemetry metric API, based on a `MeterImpl`
-type Meter interface {
+// implementation and the `Meter` library name.
+//
+// An uninitialized Meter is a no-op implementation.
+type Meter struct {
+	impl        MeterImpl
+	libraryName string
+}
+
 // RecordBatch atomically records a batch of measurements.
-	RecordBatch(context.Context, []core.KeyValue, ...Measurement)
+func (m Meter) RecordBatch(ctx context.Context, ls []core.KeyValue, ms ...Measurement) {
+	if m.impl == nil {
+		return
+	}
+	m.impl.RecordBatch(ctx, ls, ms...)
+}
 
-	// All instrument constructors may return an error for
+// NewInt64Counter creates a new integer Counter instrument with the
-	// conditions such as:
+// given name, customized with options.  May return an error if the
-	//   `name` is an empty string
+// name is invalid (e.g., empty) or improperly registered (e.g.,
-	//   `name` was previously registered as a different kind of instrument
+// duplicate registration).
-	//          for a given named `Meter`.
+func (m Meter) NewInt64Counter(name string, options ...Option) (Int64Counter, error) {
+	return wrapInt64CounterInstrument(
+		m.newSync(name, CounterKind, Int64NumberKind, options))
+}
 
-	// NewInt64Counter creates a new integral counter with a given
+// NewFloat64Counter creates a new floating point Counter with the
-	// name and customized with passed options.
+// given name, customized with options.  May return an error if the
-	NewInt64Counter(name string, opts ...Option) (Int64Counter, error)
+// name is invalid (e.g., empty) or improperly registered (e.g.,
-	// NewFloat64Counter creates a new floating point counter with
+// duplicate registration).
-	// a given name and customized with passed options.
+func (m Meter) NewFloat64Counter(name string, options ...Option) (Float64Counter, error) {
-	NewFloat64Counter(name string, opts ...Option) (Float64Counter, error)
+	return wrapFloat64CounterInstrument(
-	// NewInt64Measure creates a new integral measure with a given
+		m.newSync(name, CounterKind, Float64NumberKind, options))
-	// name and customized with passed options.
+}
-	NewInt64Measure(name string, opts ...Option) (Int64Measure, error)
-	// NewFloat64Measure creates a new floating point measure with
-	// a given name and customized with passed options.
-	NewFloat64Measure(name string, opts ...Option) (Float64Measure, error)
 
-	// RegisterInt64Observer creates a new integral observer with a
+// NewInt64Measure creates a new integer Measure instrument with the
-	// given name, running a given callback, and customized with passed
+// given name, customized with options.  May return an error if the
-	// options. Callback may be nil.
+// name is invalid (e.g., empty) or improperly registered (e.g.,
-	RegisterInt64Observer(name string, callback Int64ObserverCallback, opts ...Option) (Int64Observer, error)
+// duplicate registration).
-	// RegisterFloat64Observer creates a new floating point observer
+func (m Meter) NewInt64Measure(name string, opts ...Option) (Int64Measure, error) {
-	// with a given name, running a given callback, and customized with
+	return wrapInt64MeasureInstrument(
-	// passed options. Callback may be nil.
+		m.newSync(name, MeasureKind, Int64NumberKind, opts))
-	RegisterFloat64Observer(name string, callback Float64ObserverCallback, opts ...Option) (Float64Observer, error)
+}
+
+// NewFloat64Measure creates a new floating point Measure with the
+// given name, customized with options.  May return an error if the
+// name is invalid (e.g., empty) or improperly registered (e.g.,
+// duplicate registration).
+func (m Meter) NewFloat64Measure(name string, opts ...Option) (Float64Measure, error) {
+	return wrapFloat64MeasureInstrument(
+		m.newSync(name, MeasureKind, Float64NumberKind, opts))
+}
+
+// RegisterInt64Observer creates a new integer Observer instrument
+// with the given name, running a given callback, and customized with
+// options.  May return an error if the name is invalid (e.g., empty)
+// or improperly registered (e.g., duplicate registration).
+func (m Meter) RegisterInt64Observer(name string, callback Int64ObserverCallback, opts ...Option) (Int64Observer, error) {
+	if callback == nil {
+		return wrapInt64ObserverInstrument(NoopAsync{}, nil)
+	}
+	return wrapInt64ObserverInstrument(
+		m.newAsync(name, ObserverKind, Int64NumberKind, opts,
+			func(observe func(Number, []core.KeyValue)) {
+				callback(Int64ObserverResult{observe})
+			}))
+}
+
+// RegisterFloat64Observer creates a new floating point Observer with
+// the given name, running a given callback, and customized with
+// options.  May return an error if the name is invalid (e.g., empty)
+// or improperly registered (e.g., duplicate registration).
+func (m Meter) RegisterFloat64Observer(name string, callback Float64ObserverCallback, opts ...Option) (Float64Observer, error) {
+	if callback == nil {
+		return wrapFloat64ObserverInstrument(NoopAsync{}, nil)
+	}
+	return wrapFloat64ObserverInstrument(
+		m.newAsync(name, ObserverKind, Float64NumberKind, opts,
+			func(observe func(Number, []core.KeyValue)) {
+				callback(Float64ObserverResult{observe})
+			}))
+}
+
+// Observe captures a single integer value from the associated
+// instrument callback, with the given labels.
+func (io Int64ObserverResult) Observe(value int64, labels ...core.KeyValue) {
+	io.observe(NewInt64Number(value), labels)
+}
+
+// Observe captures a single floating point value from the associated
+// instrument callback, with the given labels.
+func (fo Float64ObserverResult) Observe(value float64, labels ...core.KeyValue) {
+	fo.observe(NewFloat64Number(value), labels)
 }
 
 // WithDescription applies provided description.

api/metric/noop.go

@@ -21,7 +21,6 @@ import (
 )
 
 type NoopProvider struct{}
-type NoopMeter struct{}
 
 type noopInstrument struct{}
 type noopBoundInstrument struct{}
@@ -29,13 +28,12 @@ type NoopSync struct{ noopInstrument }
 type NoopAsync struct{ noopInstrument }
 
 var _ Provider = NoopProvider{}
-var _ Meter = NoopMeter{}
 var _ SyncImpl = NoopSync{}
 var _ BoundSyncImpl = noopBoundInstrument{}
 var _ AsyncImpl = NoopAsync{}
 
 func (NoopProvider) Meter(name string) Meter {
-	return NoopMeter{}
+	return Meter{}
 }
 
 func (noopInstrument) Implementation() interface{} {
@@ -58,30 +56,3 @@ func (NoopSync) Bind([]core.KeyValue) BoundSyncImpl {
 
 func (NoopSync) RecordOne(context.Context, Number, []core.KeyValue) {
 }
-
-func (NoopMeter) RecordBatch(context.Context, []core.KeyValue, ...Measurement) {
-}
-
-func (NoopMeter) NewInt64Counter(string, ...Option) (Int64Counter, error) {
-	return Int64Counter{syncInstrument{NoopSync{}}}, nil
-}
-
-func (NoopMeter) NewFloat64Counter(string, ...Option) (Float64Counter, error) {
-	return Float64Counter{syncInstrument{NoopSync{}}}, nil
-}
-
-func (NoopMeter) NewInt64Measure(string, ...Option) (Int64Measure, error) {
-	return Int64Measure{syncInstrument{NoopSync{}}}, nil
-}
-
-func (NoopMeter) NewFloat64Measure(string, ...Option) (Float64Measure, error) {
-	return Float64Measure{syncInstrument{NoopSync{}}}, nil
-}
-
-func (NoopMeter) RegisterInt64Observer(string, Int64ObserverCallback, ...Option) (Int64Observer, error) {
-	return Int64Observer{asyncInstrument{NoopAsync{}}}, nil
-}
-
-func (NoopMeter) RegisterFloat64Observer(string, Float64ObserverCallback, ...Option) (Float64Observer, error) {
-	return Float64Observer{asyncInstrument{NoopAsync{}}}, nil
-}

api/metric/observer.go

@@ -14,20 +14,6 @@
 
 package metric
 
-import "go.opentelemetry.io/otel/api/core"
-
-// Int64ObserverResult is an interface for reporting integral
-// observations.
-type Int64ObserverResult interface {
-	Observe(value int64, labels ...core.KeyValue)
-}
-
-// Float64ObserverResult is an interface for reporting floating point
-// observations.
-type Float64ObserverResult interface {
-	Observe(value float64, labels ...core.KeyValue)
-}
-
 // Int64ObserverCallback is a type of callback that integral
 // observers run.
 type Int64ObserverCallback func(result Int64ObserverResult)

api/metric/sdkhelpers.go

@@ -20,10 +20,8 @@ import (
 	"go.opentelemetry.io/otel/api/core"
 )
 
-// MeterImpl is a convenient interface for SDK and test
+// MeterImpl is the interface an SDK must implement to supply a Meter
-// implementations that would provide a `Meter` but do not wish to
+// implementation.
-// re-implement the API's type-safe interfaces.  Helpers provided in
-// this package will construct a `Meter` given a `MeterImpl`.
 type MeterImpl interface {
 	// RecordBatch atomically records a batch of measurements.
 	RecordBatch(context.Context, []core.KeyValue, ...Measurement)
@@ -83,35 +81,20 @@ type BoundSyncImpl interface {
 // asynchronous instrument (e.g., Observer instruments).
 type AsyncImpl interface {
 	InstrumentImpl
-
-	// Note: An `Unregister()` API could be supported here.
 }
 
-// wrappedMeterImpl implements the `Meter` interface given a
+// Int64ObserverResult is passed to an observer callback to capture
-// `MeterImpl` implementation.
+// observations for one asynchronous integer metric instrument.
-type wrappedMeterImpl struct {
+type Int64ObserverResult struct {
-	impl        MeterImpl
-	libraryName string
-}
-
-// int64ObserverResult is an adapter for int64-valued asynchronous
-// callbacks.
-type int64ObserverResult struct {
 	observe func(Number, []core.KeyValue)
 }
 
-// float64ObserverResult is an adapter for float64-valued asynchronous
+// Float64ObserverResult is passed to an observer callback to capture
-// callbacks.
+// observations for one asynchronous floating point metric instrument.
-type float64ObserverResult struct {
+type Float64ObserverResult struct {
 	observe func(Number, []core.KeyValue)
 }
 
-var (
-	_ Meter                 = (*wrappedMeterImpl)(nil)
-	_ Int64ObserverResult   = int64ObserverResult{}
-	_ Float64ObserverResult = float64ObserverResult{}
-)
-
 // Configure is a helper that applies all the options to a Config.
 func Configure(opts []Option) Config {
 	var config Config
@@ -124,131 +107,87 @@ func Configure(opts []Option) Config {
 // WrapMeterImpl constructs a `Meter` implementation from a
 // `MeterImpl` implementation.
 func WrapMeterImpl(impl MeterImpl, libraryName string) Meter {
-	return &wrappedMeterImpl{
+	return Meter{
 		impl:        impl,
 		libraryName: libraryName,
 	}
 }
 
-func (m *wrappedMeterImpl) RecordBatch(ctx context.Context, ls []core.KeyValue, ms ...Measurement) {
+// MeterImpl returns the underlying MeterImpl of this Meter.
-	m.impl.RecordBatch(ctx, ls, ms...)
+func (m Meter) MeterImpl() MeterImpl {
+	return m.impl
 }
 
-func (m *wrappedMeterImpl) newSync(name string, metricKind Kind, numberKind NumberKind, opts []Option) (SyncImpl, error) {
+// newSync constructs one new synchronous instrument.
+func (m Meter) newSync(name string, metricKind Kind, numberKind NumberKind, opts []Option) (SyncImpl, error) {
+	if m.impl == nil {
+		return NoopSync{}, nil
+	}
 	desc := NewDescriptor(name, metricKind, numberKind, opts...)
 	desc.config.LibraryName = m.libraryName
 	return m.impl.NewSyncInstrument(desc)
 }
 
-func (m *wrappedMeterImpl) NewInt64Counter(name string, opts ...Option) (Int64Counter, error) {
+// wrapInt64CounterInstrument returns an `Int64Counter` from a
-	return WrapInt64CounterInstrument(
-		m.newSync(name, CounterKind, Int64NumberKind, opts))
-}
-
-// WrapInt64CounterInstrument returns an `Int64Counter` from a
 // `SyncImpl`.  An error will be generated if the
 // `SyncImpl` is nil (in which case a No-op is substituted),
 // otherwise the error passes through.
-func WrapInt64CounterInstrument(syncInst SyncImpl, err error) (Int64Counter, error) {
+func wrapInt64CounterInstrument(syncInst SyncImpl, err error) (Int64Counter, error) {
 	common, err := checkNewSync(syncInst, err)
 	return Int64Counter{syncInstrument: common}, err
 }
 
-func (m *wrappedMeterImpl) NewFloat64Counter(name string, opts ...Option) (Float64Counter, error) {
+// wrapFloat64CounterInstrument returns an `Float64Counter` from a
-	return WrapFloat64CounterInstrument(
-		m.newSync(name, CounterKind, Float64NumberKind, opts))
-}
-
-// WrapFloat64CounterInstrument returns an `Float64Counter` from a
 // `SyncImpl`.  An error will be generated if the
 // `SyncImpl` is nil (in which case a No-op is substituted),
 // otherwise the error passes through.
-func WrapFloat64CounterInstrument(syncInst SyncImpl, err error) (Float64Counter, error) {
+func wrapFloat64CounterInstrument(syncInst SyncImpl, err error) (Float64Counter, error) {
 	common, err := checkNewSync(syncInst, err)
 	return Float64Counter{syncInstrument: common}, err
 }
 
-func (m *wrappedMeterImpl) NewInt64Measure(name string, opts ...Option) (Int64Measure, error) {
+// wrapInt64MeasureInstrument returns an `Int64Measure` from a
-	return WrapInt64MeasureInstrument(
-		m.newSync(name, MeasureKind, Int64NumberKind, opts))
-}
-
-// WrapInt64MeasureInstrument returns an `Int64Measure` from a
 // `SyncImpl`.  An error will be generated if the
 // `SyncImpl` is nil (in which case a No-op is substituted),
 // otherwise the error passes through.
-func WrapInt64MeasureInstrument(syncInst SyncImpl, err error) (Int64Measure, error) {
+func wrapInt64MeasureInstrument(syncInst SyncImpl, err error) (Int64Measure, error) {
 	common, err := checkNewSync(syncInst, err)
 	return Int64Measure{syncInstrument: common}, err
 }
 
-func (m *wrappedMeterImpl) NewFloat64Measure(name string, opts ...Option) (Float64Measure, error) {
+// wrapFloat64MeasureInstrument returns an `Float64Measure` from a
-	return WrapFloat64MeasureInstrument(
-		m.newSync(name, MeasureKind, Float64NumberKind, opts))
-}
-
-// WrapFloat64MeasureInstrument returns an `Float64Measure` from a
 // `SyncImpl`.  An error will be generated if the
 // `SyncImpl` is nil (in which case a No-op is substituted),
 // otherwise the error passes through.
-func WrapFloat64MeasureInstrument(syncInst SyncImpl, err error) (Float64Measure, error) {
+func wrapFloat64MeasureInstrument(syncInst SyncImpl, err error) (Float64Measure, error) {
 	common, err := checkNewSync(syncInst, err)
 	return Float64Measure{syncInstrument: common}, err
 }
 
-func (m *wrappedMeterImpl) newAsync(name string, mkind Kind, nkind NumberKind, opts []Option, callback func(func(Number, []core.KeyValue))) (AsyncImpl, error) {
+// newAsync constructs one new asynchronous instrument.
+func (m Meter) newAsync(name string, mkind Kind, nkind NumberKind, opts []Option, callback func(func(Number, []core.KeyValue))) (AsyncImpl, error) {
+	if m.impl == nil {
+		return NoopAsync{}, nil
+	}
 	desc := NewDescriptor(name, mkind, nkind, opts...)
 	desc.config.LibraryName = m.libraryName
 	return m.impl.NewAsyncInstrument(desc, callback)
 }
 
-func (m *wrappedMeterImpl) RegisterInt64Observer(name string, callback Int64ObserverCallback, opts ...Option) (Int64Observer, error) {
+// wrapInt64ObserverInstrument returns an `Int64Observer` from a
-	if callback == nil {
-		return NoopMeter{}.RegisterInt64Observer("", nil)
-	}
-	return WrapInt64ObserverInstrument(
-		m.newAsync(name, ObserverKind, Int64NumberKind, opts,
-			func(observe func(Number, []core.KeyValue)) {
-				// Note: this memory allocation could be avoided by
-				// using a pointer to this object and mutating it
-				// on each collection interval.
-				callback(int64ObserverResult{observe})
-			}))
-}
-
-// WrapInt64ObserverInstrument returns an `Int64Observer` from a
 // `AsyncImpl`.  An error will be generated if the
 // `AsyncImpl` is nil (in which case a No-op is substituted),
 // otherwise the error passes through.
-func WrapInt64ObserverInstrument(asyncInst AsyncImpl, err error) (Int64Observer, error) {
+func wrapInt64ObserverInstrument(asyncInst AsyncImpl, err error) (Int64Observer, error) {
 	common, err := checkNewAsync(asyncInst, err)
 	return Int64Observer{asyncInstrument: common}, err
 }
 
-func (m *wrappedMeterImpl) RegisterFloat64Observer(name string, callback Float64ObserverCallback, opts ...Option) (Float64Observer, error) {
+// wrapFloat64ObserverInstrument returns an `Float64Observer` from a
-	if callback == nil {
-		return NoopMeter{}.RegisterFloat64Observer("", nil)
-	}
-	return WrapFloat64ObserverInstrument(
-		m.newAsync(name, ObserverKind, Float64NumberKind, opts,
-			func(observe func(Number, []core.KeyValue)) {
-				callback(float64ObserverResult{observe})
-			}))
-}
-
-// WrapFloat64ObserverInstrument returns an `Float64Observer` from a
 // `AsyncImpl`.  An error will be generated if the
 // `AsyncImpl` is nil (in which case a No-op is substituted),
 // otherwise the error passes through.
-func WrapFloat64ObserverInstrument(asyncInst AsyncImpl, err error) (Float64Observer, error) {
+func wrapFloat64ObserverInstrument(asyncInst AsyncImpl, err error) (Float64Observer, error) {
 	common, err := checkNewAsync(asyncInst, err)
 	return Float64Observer{asyncInstrument: common}, err
 }
-
-func (io int64ObserverResult) Observe(value int64, labels ...core.KeyValue) {
-	io.observe(NewInt64Number(value), labels)
-}
-
-func (fo float64ObserverResult) Observe(value float64, labels ...core.KeyValue) {
-	fo.observe(NewFloat64Number(value), labels)
-}