Propagating OpenTelemetry trace context through the HTTP to GRPC gateway to the request handlers

demo.go

package demo

import (
	"context"
	"net/http"

	"github.com/grpc-ecosystem/grpc-gateway/v2/runtime"
	"go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp"
	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/propagation"
	"google.golang.org/grpc"
	"google.golang.org/grpc/metadata"
)

// propagateTraceContext injects the trace context into the request metadata.
// You'll also need otelgrpc.UnaryServerInterceptor. See their example:
// https://github.com/open-telemetry/opentelemetry-go-contrib/tree/main/instrumentation/google.golang.org/grpc/otelgrpc/example
func propagateTraceContext(ctx context.Context, req *http.Request) metadata.MD {
	carrier := propagation.MapCarrier{}
	otel.GetTextMapPropagator().Inject(ctx, carrier)
	return metadata.New(carrier)
}

func grpcServiceMux(ctx context.Context, endpoint string, opts ...grpc.DialOption) (http.Handler, error) {
	mux := runtime.NewServeMux(runtime.WithMetadata(propagateTraceContext))

	if err := RegisterMyServiceHandlerFromEndpoint(ctx, mux, endpoint, opts); err != nil {
		return err
	}

	return otelhttp.NewHandler(mux, "HTTP"), nil
}