Golang opentelemetry-go draft API (incomplete) (#9)

* Work in progress from https://github.com/lightstep/opentelemetry-golang-prototype

* Renames

* Rename

* Finish rename

* Rename packages

* README

api/metric/common.go

@@ -0,0 +1,74 @@
+package metric
+
+import (
+	"github.com/open-telemetry/opentelemetry-go/api/core"
+	"github.com/open-telemetry/opentelemetry-go/api/scope"
+	"github.com/open-telemetry/opentelemetry-go/api/tag"
+	"github.com/open-telemetry/opentelemetry-go/exporter/observer"
+)
+
+type (
+	baseMetric struct {
+		measure core.Measure
+
+		mtype   MetricType
+		keys    []core.Key
+		eventID core.EventID
+		status  error // Indicates registry conflict
+	}
+
+	baseEntry struct {
+		base    *baseMetric
+		metric  Metric
+		eventID core.EventID
+	}
+)
+
+func initBaseMetric(name string, mtype MetricType, opts []Option, init Metric) Metric {
+	var tagOpts []tag.Option
+	bm := init.base()
+
+	for _, opt := range opts {
+		opt(bm, &tagOpts)
+	}
+
+	bm.measure = tag.NewMeasure(name, tagOpts...)
+	bm.mtype = mtype
+
+	bm.eventID = observer.Record(observer.Event{
+		Type:  observer.NEW_METRIC,
+		Scope: bm.measure.DefinitionID().Scope(),
+	})
+
+	other, err := GetRegistry().RegisterMetric(init)
+	if err != nil {
+		bm.status = err
+	}
+	return other
+}
+
+func (bm *baseMetric) base() *baseMetric {
+	return bm
+}
+
+func (bm *baseMetric) Measure() core.Measure {
+	return bm.measure
+}
+
+func (bm *baseMetric) Type() MetricType {
+	return bm.mtype
+}
+
+func (bm *baseMetric) Fields() []core.Key {
+	return bm.keys
+}
+
+func (bm *baseMetric) Err() error {
+	return bm.status
+}
+
+func (e *baseEntry) init(m Metric, values []core.KeyValue) {
+	e.base = m.base()
+	e.metric = m
+	e.eventID = scope.New(core.ScopeID{}, values...).ScopeID().EventID
+}