Fixes the instrument kind for noop async instruments

Signed-off-by: Bogdan Drutu <bogdandrutu@gmail.com>

CHANGELOG.md

@@ -14,6 +14,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
   - Import path changed `import "go.opentelemetry.io/otel/sdk/export/metric"` to `import go.opentelemetry.io/otel/sdk/metric/export` (#2382).
 - Deprecate `AtomicFieldOffsets`, unnecessary public func (#2445)
 
+### Fixed
+
+- Fixes the instrument kind for noop async instruments (#2461)
+
 ## [1.3.0] - 2021-12-10
 
 ### ⚠️ Notice ⚠️

internal/metric/registry/registry_test.go

@@ -19,6 +19,7 @@ import (
 	"errors"
 	"testing"
 
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
 	"go.opentelemetry.io/otel/internal/metric/registry"
@@ -125,3 +126,13 @@ func TestRegistryDiffInstruments(t *testing.T) {
 		}
 	}
 }
+
+func TestDoubleNewInstrument(t *testing.T) {
+	reg := registry.NewUniqueInstrumentMeterImpl(
+		metrictest.NewMeterProvider().Meter("meter").MeterImpl(),
+	)
+	_, err := reg.NewAsyncInstrument(sdkapi.NewNoopAsyncInstrument().Descriptor(), nil)
+	require.NoError(t, err)
+	_, err = reg.NewSyncInstrument(sdkapi.NewNoopSyncInstrument().Descriptor())
+	assert.Error(t, err)
+}

metric/sdkapi/noop.go

@@ -21,7 +21,9 @@ import (
 	"go.opentelemetry.io/otel/metric/number"
 )
 
-type noopInstrument struct{}
+type noopInstrument struct {
+	descriptor Descriptor
+}
 type noopSyncInstrument struct{ noopInstrument }
 type noopAsyncInstrument struct{ noopInstrument }
 
@@ -31,21 +33,33 @@ var _ AsyncImpl = noopAsyncInstrument{}
 // NewNoopSyncInstrument returns a No-op implementation of the
 // synchronous instrument interface.
 func NewNoopSyncInstrument() SyncImpl {
-	return noopSyncInstrument{}
+	return noopSyncInstrument{
+		noopInstrument{
+			descriptor: Descriptor{
+				instrumentKind: CounterInstrumentKind,
+			},
+		},
+	}
 }
 
 // NewNoopAsyncInstrument returns a No-op implementation of the
 // asynchronous instrument interface.
 func NewNoopAsyncInstrument() AsyncImpl {
-	return noopAsyncInstrument{}
+	return noopAsyncInstrument{
+		noopInstrument{
+			descriptor: Descriptor{
+				instrumentKind: CounterObserverInstrumentKind,
+			},
+		},
+	}
 }
 
 func (noopInstrument) Implementation() interface{} {
 	return nil
 }
 
-func (noopInstrument) Descriptor() Descriptor {
-	return Descriptor{}
+func (n noopInstrument) Descriptor() Descriptor {
+	return n.descriptor
 }
 
 func (noopSyncInstrument) RecordOne(context.Context, number.Number, []attribute.KeyValue) {