otelgrpc: add MetricAttributesFn option to allow adding dynamic attributes on metrics

CHANGELOG.md

@@ -20,6 +20,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Add unmarshaling and validation for `OTLPHttpExporter`, `OTLPGrpcExporter`, `OTLPGrpcMetricExporter` and `OTLPHttpMetricExporter` to v1.0.0 model in `go.opentelemetry.io/contrib/otelconf`. (#8112)
 - Add a `WithSpanNameFormatter` option to `go.opentelemetry.io/contrib/instrumentation/go.mongodb.org/mongo-driver/v2/mongo/otelmongo`. (#7986)
 - Add unmarshaling and validation for `AttributeType`, `AttributeNameValue`, `SimpleSpanProcessor`, `SimpleLogRecordProcessor`, `ZipkinSpanExporter`, `NameStringValuePair`, `InstrumentType`, `ExperimentalPeerInstrumentationServiceMappingElem`, `ExporterDefaultHistogramAggregation`, `PullMetricReader` to v1.0.0 model in `go.opentelemetry.io/contrib/otelconf`. (#8127)
+- `WithMetricAttributesFn` option in `go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc` to define dynamic attributes on auto-instrumented metrics. (#8191)
 
 ### Changed
 

instrumentation/google.golang.org/grpc/otelgrpc/config.go

@@ -31,14 +31,15 @@ type Filter func(*stats.RPCTagInfo) bool
 
 // config is a group of options for this instrumentation.
 type config struct {
-	Filter            Filter
-	InterceptorFilter InterceptorFilter
-	Propagators       propagation.TextMapPropagator
-	TracerProvider    trace.TracerProvider
-	MeterProvider     metric.MeterProvider
-	SpanStartOptions  []trace.SpanStartOption
-	SpanAttributes    []attribute.KeyValue
-	MetricAttributes  []attribute.KeyValue
+	Filter             Filter
+	InterceptorFilter  InterceptorFilter
+	Propagators        propagation.TextMapPropagator
+	TracerProvider     trace.TracerProvider
+	MeterProvider      metric.MeterProvider
+	SpanStartOptions   []trace.SpanStartOption
+	SpanAttributes     []attribute.KeyValue
+	MetricAttributes   []attribute.KeyValue
+	MetricAttributesFn func(ctx context.Context) []attribute.KeyValue
 
 	PublicEndpoint   bool
 	PublicEndpointFn func(ctx context.Context, info *stats.RPCTagInfo) bool
@@ -193,3 +194,18 @@ func WithMetricAttributes(a ...attribute.KeyValue) Option {
 		}
 	})
 }
+
+type metricAttributesFnOption struct {
+	fn func(ctx context.Context) []attribute.KeyValue
+}
+
+func (o metricAttributesFnOption) apply(c *config) {
+	if o.fn != nil {
+		c.MetricAttributesFn = o.fn
+	}
+}
+
+// WithMetricAttributesFn returns an Option to add custom attributes to the metrics.
+func WithMetricAttributesFn(fn func(ctx context.Context) []attribute.KeyValue) Option {
+	return metricAttributesFnOption{fn: fn}
+}

instrumentation/google.golang.org/grpc/otelgrpc/example_test.go

@@ -4,7 +4,11 @@
 package otelgrpc_test
 
 import (
+	"context"
+
+	"go.opentelemetry.io/otel/attribute"
 	"google.golang.org/grpc"
+	"google.golang.org/grpc/metadata"
 
 	"go.opentelemetry.io/contrib/instrumentation/google.golang.org/grpc/otelgrpc"
 )
@@ -16,3 +20,48 @@ func ExampleNewClientHandler() {
 func ExampleNewServerHandler() {
 	_ = grpc.NewServer(grpc.StatsHandler(otelgrpc.NewServerHandler()))
 }
+
+// ExampleNewClientHandler_withMetricAttributesFn demonstrates how to add a dynamic
+// client-side attribute to the auto-instrumented metrics.
+func ExampleNewClientHandler_withMetricAttributesFn() {
+	// should be centralized, example only
+	type retryAttemptKey struct{}
+
+	// a middleware must populate that key with the actual retry attempt, e.g.,
+	// ...
+	// ctx := context.WithValue(context.Background(), retryAttemptKey{}, attempt)
+	// ...
+
+	_, _ = grpc.NewClient("localhost", grpc.WithStatsHandler(otelgrpc.NewClientHandler(
+		otelgrpc.WithMetricAttributesFn(func(ctx context.Context) []attribute.KeyValue {
+			if attempt, ok := ctx.Value(retryAttemptKey{}).(int); ok {
+				return []attribute.KeyValue{
+					// Caution: example only.
+					// This must be a controlled, bounded and very limited set of numbers
+					// so that you don't end up with very high cardinality.
+					attribute.Int("retry.attempt", attempt),
+				}
+			}
+
+			return nil
+		}),
+	)))
+}
+
+// ExampleNewClientHandler_withMetricAttributesFn demonstrates how to add a dynamic
+// server-side attribute to the auto-instrumented metrics.
+func ExampleNewServerHandler_withMetricAttributesFn() {
+	_ = grpc.NewServer(grpc.StatsHandler(otelgrpc.NewServerHandler(
+		otelgrpc.WithMetricAttributesFn(func(ctx context.Context) []attribute.KeyValue {
+			md, ok := metadata.FromIncomingContext(ctx)
+			if ok {
+				if origins := md.Get("origin"); len(origins) > 0 && origins[0] != "" {
+					return []attribute.KeyValue{attribute.String("origin", origins[0])}
+				}
+			}
+
+			// Some use-cases might require to fallback to a default.
+			return []attribute.KeyValue{attribute.String("origin", "unknown")}
+		}),
+	)))
+}