Added a Callback option to the api

metric/example_test.go

@@ -22,6 +22,7 @@ import (
 
 	"go.opentelemetry.io/otel/metric"
 	"go.opentelemetry.io/otel/metric/instrument"
+	"go.opentelemetry.io/otel/metric/instrument/asyncint64"
 	"go.opentelemetry.io/otel/metric/instrument/syncfloat64"
 	"go.opentelemetry.io/otel/metric/unit"
 )
@@ -51,26 +52,19 @@ func ExampleAsyncInstrument() {
 	meterProvider := metric.NewNoopMeterProvider()
 	meter := meterProvider.Meter("AsyncExample")
 
-	memoryUsage, err := meter.AsyncInt64().Gauge(
+	_, err := meter.AsyncInt64().Gauge(
 		"MemoryUsage",
 		instrument.WithUnit(unit.Bytes),
-	)
-	if err != nil {
-		fmt.Println("Failed to register instrument")
-		panic(err)
-	}
-
-	err = meter.RegisterCallback([]instrument.Asynchronous{memoryUsage},
-		func(ctx context.Context) {
-			// instrument.WithCallbackFunc(func(ctx context.Context) {
+		asyncint64.WithGaugeCallback(func(ctx context.Context, memoryUsage asyncint64.Gauge) {
 			//Do Work to get the real memoryUsage
 			// mem := GatherMemory(ctx)
 			mem := 75000
 
 			memoryUsage.Observe(ctx, int64(mem))
-		})
+		}),
+	)
 	if err != nil {
-		fmt.Println("Failed to register callback")
+		fmt.Println("Failed to register instrument")
 		panic(err)
 	}
 }

metric/instrument/asyncfloat64/asyncfloat64.go

@@ -68,3 +68,25 @@ type Gauge interface {
 
 	instrument.Asynchronous
 }
+
+func WithCounterCallback(fn func(context.Context, Counter)) instrument.Option {
+	return instrument.WithCallback(func(ctx context.Context, inst instrument.Asynchronous) {
+		if counter, ok := inst.(Counter); ok {
+			fn(ctx, counter)
+		}
+	})
+}
+func WithUpDownCounterCallback(fn func(context.Context, UpDownCounter)) instrument.Option {
+	return instrument.WithCallback(func(ctx context.Context, inst instrument.Asynchronous) {
+		if counter, ok := inst.(UpDownCounter); ok {
+			fn(ctx, counter)
+		}
+	})
+}
+func WithGaugeCallback(fn func(context.Context, Gauge)) instrument.Option {
+	return instrument.WithCallback(func(ctx context.Context, inst instrument.Asynchronous) {
+		if gauge, ok := inst.(Gauge); ok {
+			fn(ctx, gauge)
+		}
+	})
+}

metric/instrument/asyncint64/asyncint64.go

@@ -68,3 +68,25 @@ type Gauge interface {
 
 	instrument.Asynchronous
 }
+
+func WithCounterCallback(fn func(context.Context, Counter)) instrument.Option {
+	return instrument.WithCallback(func(ctx context.Context, inst instrument.Asynchronous) {
+		if counter, ok := inst.(Counter); ok {
+			fn(ctx, counter)
+		}
+	})
+}
+func WithUpDownCounterCallback(fn func(context.Context, UpDownCounter)) instrument.Option {
+	return instrument.WithCallback(func(ctx context.Context, inst instrument.Asynchronous) {
+		if counter, ok := inst.(UpDownCounter); ok {
+			fn(ctx, counter)
+		}
+	})
+}
+func WithGaugeCallback(fn func(context.Context, Gauge)) instrument.Option {
+	return instrument.WithCallback(func(ctx context.Context, inst instrument.Asynchronous) {
+		if gauge, ok := inst.(Gauge); ok {
+			fn(ctx, gauge)
+		}
+	})
+}

metric/instrument/config.go

@@ -14,12 +14,17 @@
 
 package instrument // import "go.opentelemetry.io/otel/metric/instrument"
 
-import "go.opentelemetry.io/otel/metric/unit"
+import (
+	"go.opentelemetry.io/otel/metric/unit"
+	"golang.org/x/net/context"
+)
 
 // Config contains options for metric instrument descriptors.
 type Config struct {
 	description string
 	unit        unit.Unit
+
+	callback func(context.Context, Asynchronous)
 }
 
 // Description describes the instrument in human-readable terms.
@@ -32,6 +37,12 @@ func (cfg Config) Unit() unit.Unit {
 	return cfg.unit
 }
 
+// Callback returns the function that will be registered on asynchronous
+// instrument creation.
+func (cfg Config) Callback() func(context.Context, Asynchronous) {
+	return cfg.callback
+}
+
 // Option is an interface for applying metric instrument options.
 type Option interface {
 	// ApplyMeter is used to set a Option value of a
@@ -68,3 +79,9 @@ func WithUnit(unit unit.Unit) Option {
 		cfg.unit = unit
 	})
 }
+
+func WithCallback(fn func(context.Context, Asynchronous)) Option {
+	return optionFunc(func(cfg *Config) {
+		cfg.callback = fn
+	})
+}