adding NewNoopMeterProvider to follow trace api (#2237)

* adding NewNoopMeterProvider to follow trace api

* Update CHANGELOG.md

Co-authored-by: Anthony Mirabella <a9@aneurysm9.com>

* fix lint

Co-authored-by: Anthony Mirabella <a9@aneurysm9.com>
Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

CHANGELOG.md

@@ -8,6 +8,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ## [Unreleased]
 
+### Changed
+
+- NoopMeterProvider is now private and NewNoopMeterProvider must be used to obtain a noopMeterProvider. (#2237)
+
 ### Fixed
 
 - Slice-valued attributes can correctly be used as map keys. (#2223)

metric/global/metric_test.go

@@ -30,12 +30,12 @@ func (*testMeterProvider) Meter(_ string, _ ...metric.MeterOption) metric.Meter
 
 func TestMultipleGlobalMeterProvider(t *testing.T) {
 	p1 := testMeterProvider{}
-	p2 := metric.NoopMeterProvider{}
+	p2 := metric.NewNoopMeterProvider()
 	SetMeterProvider(&p1)
-	SetMeterProvider(&p2)
+	SetMeterProvider(p2)
 
 	got := GetMeterProvider()
-	want := &p2
+	want := p2
 	if got != want {
 		t.Fatalf("MeterProvider: got %p, want %p\n", got, want)
 	}

metric/metric_noop.go → metric/noop.go

@@ -21,19 +21,26 @@ import (
 	"go.opentelemetry.io/otel/metric/number"
 )
 
-type NoopMeterProvider struct{}
+// NewNoopMeterProvider returns an implementation of MeterProvider that
+// performs no operations. The Meter and Instrument created from the returned
+// MeterProvider also perform no operations.
+func NewNoopMeterProvider() MeterProvider {
+	return noopMeterProvider{}
+}
+
+type noopMeterProvider struct{}
 
 type noopInstrument struct{}
 type noopBoundInstrument struct{}
 type NoopSync struct{ noopInstrument }
 type NoopAsync struct{ noopInstrument }
 
-var _ MeterProvider = NoopMeterProvider{}
+var _ MeterProvider = noopMeterProvider{}
 var _ SyncImpl = NoopSync{}
 var _ BoundSyncImpl = noopBoundInstrument{}
 var _ AsyncImpl = NoopAsync{}
 
-func (NoopMeterProvider) Meter(_ string, _ ...MeterOption) Meter {
+func (noopMeterProvider) Meter(instrumentationName string, opts ...MeterOption) Meter {
 	return Meter{}
 }
 

metric/noop_test.go

@@ -0,0 +1,34 @@
+// Copyright The 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
+
+import (
+	"testing"
+)
+
+func TestNewNoopMeterProvider(t *testing.T) {
+	got, want := NewNoopMeterProvider(), noopMeterProvider{}
+	if got != want {
+		t.Errorf("NewNoopMeterProvider() returned %#v, want %#v", got, want)
+	}
+}
+
+func TestNoopMeterProviderMeter(t *testing.T) {
+	mp := NewNoopMeterProvider()
+	got, want := mp.Meter(""), Meter{}
+	if got != want {
+		t.Errorf("noopMeterProvider.Meter() returned %#v, want %#v", got, want)
+	}
+}