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experimental/stats: Expose Telemetry Label Callback #8877

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6dda7c7
[i-8682][WIP] expose telemetry label callback
seth-epps Feb 2, 2026
12bf10e
vet
seth-epps Feb 3, 2026
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59 changes: 59 additions & 0 deletions experimental/stats/telemetry.go
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Original file line number Diff line number Diff line change
@@ -0,0 +1,59 @@
/*
*
* Copyright 2026 gRPC 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 stats

import (
"context"

"google.golang.org/grpc/grpclog"
)

// LabelCallback is a function that is executed when telemetry
// label keys are updated
type LabelCallback func(string, string)

type telemetryLabelCallbackKey struct{}

// WithTelemetryLabelCallback registers a callback function that is executed whenever
// telemetry labels will be updated. This does _not_ require opentelemetry instrumentation
// to be configured on the client or server.
func WithTelemetryLabelCallback(ctx context.Context, callback LabelCallback) context.Context {
if callback == nil {
return ctx
}
return context.WithValue(ctx, telemetryLabelCallbackKey{}, callback)
}

// ExecuteTelemetryLabelCallback runs the registered callback on the context with the provided
// key and value. If no callback is registered it does nothing.
//
// If the registered callback panics it will be swallowed and logged
func ExecuteTelemetryLabelCallback(ctx context.Context, key, value string) {
if ctx == nil {
return
}
if callback, ok := ctx.Value(telemetryLabelCallbackKey{}).(LabelCallback); ok {
defer func() {
if r := recover(); r != nil {
grpclog.Component("experimental-stats").Warningf("LabelCallback panicked: %v", r)
}
}()
callback(key, value)
}
}
87 changes: 87 additions & 0 deletions experimental/stats/telemetry_test.go
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Original file line number Diff line number Diff line change
@@ -0,0 +1,87 @@
/*
*
* Copyright 2026 gRPC 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 stats

import (
"context"
"testing"
"time"

"github.com/google/go-cmp/cmp"
)

// TestTelemetryLabels tests registering a callback function with the context and
// the effects of executing the callback on a local label state tracker. Each test
// case constructs a new context with the provided callback registered.
func (s) TestTelemetryLabels(t *testing.T) {
commonLabelValues := map[string]string{"grpc.lb.backend_service": "grpc.lb.backend_service_val", "grpc.lb.locality_val": "grpc.lb.locality_val"}
tracker := map[string]string{}

tests := map[string]struct {
callback func(string, string)
additionalLabels map[string]string
wantLabels map[string]string
}{
"NilCallback": {
callback: nil,
wantLabels: map[string]string{},
},
"NoOPCallback": {
callback: func(string, string) {},
wantLabels: map[string]string{},
},
"PanicCallback": {
callback: func(string, string) { panic("intentional panic") },
wantLabels: map[string]string{},
},
"MutatingCallback": {
callback: func(key, value string) {
tracker[key] = value
},
wantLabels: map[string]string{"grpc.lb.backend_service": "grpc.lb.backend_service_val", "grpc.lb.locality_val": "grpc.lb.locality_val"},
},
"OverrideLabelsWithCallback": {
callback: func(key, value string) {
tracker[key] = value
},
additionalLabels: map[string]string{"grpc.lb.backend_service": "grpc.lb.backend_service_other_val"},
wantLabels: map[string]string{"grpc.lb.backend_service": "grpc.lb.backend_service_other_val", "grpc.lb.locality_val": "grpc.lb.locality_val"},
},
}
for name, test := range tests {
t.Run(name, func(t *testing.T) {
// resest the tracker at the end of every test
ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
t.Cleanup(func() {
tracker = map[string]string{}
cancel()
})
ctx = WithTelemetryLabelCallback(ctx, test.callback)
for k, v := range commonLabelValues {
ExecuteTelemetryLabelCallback(ctx, k, v)
}
for k, v := range test.additionalLabels {
ExecuteTelemetryLabelCallback(ctx, k, v)
}
if diff := cmp.Diff(tracker, test.wantLabels); diff != "" {
t.Fatalf("tracked labels did not match expcted values (-got, +want): %v", diff)
}
})
}
}
3 changes: 3 additions & 0 deletions internal/xds/balancer/clusterimpl/picker.go
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Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,7 @@ import (
"google.golang.org/grpc/balancer"
"google.golang.org/grpc/codes"
"google.golang.org/grpc/connectivity"
estats "google.golang.org/grpc/experimental/stats"
"google.golang.org/grpc/internal/stats"
"google.golang.org/grpc/internal/wrr"
xdsinternal "google.golang.org/grpc/internal/xds"
Expand Down Expand Up @@ -164,6 +165,8 @@ func (d *picker) Pick(info balancer.PickInfo) (balancer.PickResult, error) {
return pr, err
}

estats.ExecuteTelemetryLabelCallback(info.Ctx, "grpc.lb.locality", xdsinternal.LocalityString(lID))
estats.ExecuteTelemetryLabelCallback(info.Ctx, "grpc.lb.backend_service", d.clusterName)
if labels != nil {
labels["grpc.lb.locality"] = xdsinternal.LocalityString(lID)
labels["grpc.lb.backend_service"] = d.clusterName
Expand Down