Update metrics API to match current spec (#228)

* Update metrics API to match current spec

* update options to match the spec

* drop the global meter API

* more docs

* get rid of leftover mentions about descriptor

api/metric/sdkhelpers.go

@@ -0,0 +1,87 @@
+// Copyright 2019, OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package metric
+
+// WrapInt64CounterInstrument wraps the instrument in the type-safe
+// wrapper as an integral counter.
+//
+// It is mostly intended for SDKs.
+func WrapInt64CounterInstrument(instrument Instrument) Int64Counter {
+	return Int64Counter{commonMetric: newCommonMetric(instrument)}
+}
+
+// WrapFloat64CounterInstrument wraps the instrument in the type-safe
+// wrapper as an floating point counter.
+//
+// It is mostly intended for SDKs.
+func WrapFloat64CounterInstrument(instrument Instrument) Float64Counter {
+	return Float64Counter{commonMetric: newCommonMetric(instrument)}
+}
+
+// WrapInt64GaugeInstrument wraps the instrument in the type-safe
+// wrapper as an integral gauge.
+//
+// It is mostly intended for SDKs.
+func WrapInt64GaugeInstrument(instrument Instrument) Int64Gauge {
+	return Int64Gauge{commonMetric: newCommonMetric(instrument)}
+}
+
+// WrapFloat64GaugeInstrument wraps the instrument in the type-safe
+// wrapper as an floating point gauge.
+//
+// It is mostly intended for SDKs.
+func WrapFloat64GaugeInstrument(instrument Instrument) Float64Gauge {
+	return Float64Gauge{commonMetric: newCommonMetric(instrument)}
+}
+
+// WrapInt64MeasureInstrument wraps the instrument in the type-safe
+// wrapper as an integral measure.
+//
+// It is mostly intended for SDKs.
+func WrapInt64MeasureInstrument(instrument Instrument) Int64Measure {
+	return Int64Measure{commonMetric: newCommonMetric(instrument)}
+}
+
+// WrapFloat64MeasureInstrument wraps the instrument in the type-safe
+// wrapper as an floating point measure.
+//
+// It is mostly intended for SDKs.
+func WrapFloat64MeasureInstrument(instrument Instrument) Float64Measure {
+	return Float64Measure{commonMetric: newCommonMetric(instrument)}
+}
+
+// ApplyCounterOptions is a helper that applies all the counter
+// options to passed opts.
+func ApplyCounterOptions(opts *Options, cos ...CounterOptionApplier) {
+	for _, o := range cos {
+		o.ApplyCounterOption(opts)
+	}
+}
+
+// ApplyGaugeOptions is a helper that applies all the gauge options to
+// passed opts.
+func ApplyGaugeOptions(opts *Options, gos ...GaugeOptionApplier) {
+	for _, o := range gos {
+		o.ApplyGaugeOption(opts)
+	}
+}
+
+// ApplyMeasureOptions is a helper that applies all the measure
+// options to passed opts.
+func ApplyMeasureOptions(opts *Options, mos ...MeasureOptionApplier) {
+	for _, o := range mos {
+		o.ApplyMeasureOption(opts)
+	}
+}