docs: add OTLP bridge tutorial for client_golang users

README.md

@@ -48,6 +48,11 @@ The
 [`examples` directory](https://github.com/prometheus/client_golang/tree/main/examples)
 contains simple examples of instrumented code.
 
+If you need to bridge existing `client_golang` instrumentation into an
+OpenTelemetry / OTLP pipeline, see the
+[`tutorials/otlp-bridge`](https://github.com/prometheus/client_golang/tree/main/tutorials/otlp-bridge)
+tutorial.
+
 ## Client for the Prometheus HTTP API
 
 [![Go Reference](https://pkg.go.dev/badge/github.com/prometheus/client_golang/api.svg)](https://pkg.go.dev/github.com/prometheus/client_golang/api)

tutorials/otlp-bridge/README.md

@@ -0,0 +1,90 @@
+# client_golang Tutorial: Export Prometheus instrumentation with OpenTelemetry OTLP
+
+`client_golang` is built around the Prometheus pull model. For most long-running
+services, exposing `/metrics` with `promhttp` is still the recommended setup.
+
+If you already instrumented your code with `client_golang` but need to send
+metrics into an OpenTelemetry pipeline, you can bridge a Prometheus
+`Gatherer` into the OpenTelemetry SDK and export it with OTLP.
+
+This is useful for:
+
+- short-lived batch jobs that may exit before being scraped
+- environments standardized on OTLP and the OpenTelemetry Collector
+- gradual migrations where existing Prometheus instrumentation should stay
+  unchanged
+
+## Example
+
+The example below keeps existing Prometheus instrumentation, bridges a custom
+registry into the OpenTelemetry SDK, and exports the resulting metrics with the
+OTLP HTTP exporter.
+
+```go
+package main
+
+import (
+	"context"
+	"log"
+
+	bridgeprometheus "go.opentelemetry.io/contrib/bridges/prometheus"
+	"go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetrichttp"
+	sdkmetric "go.opentelemetry.io/otel/sdk/metric"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+func main() {
+	ctx := context.Background()
+
+	reg := prometheus.NewRegistry()
+	jobsProcessed := prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "jobs_processed_total",
+		Help: "Total number of processed jobs.",
+	})
+	reg.MustRegister(jobsProcessed)
+
+	exporter, err := otlpmetrichttp.New(
+		ctx,
+		otlpmetrichttp.WithEndpoint("localhost:4318"),
+		otlpmetrichttp.WithInsecure(),
+	)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	bridge := bridgeprometheus.NewMetricProducer(
+		bridgeprometheus.WithGatherer(reg),
+	)
+
+	meterProvider := sdkmetric.NewMeterProvider(
+		sdkmetric.WithReader(
+			sdkmetric.NewPeriodicReader(exporter, sdkmetric.WithProducer(bridge)),
+		),
+	)
+	defer func() {
+		if err := meterProvider.Shutdown(ctx); err != nil {
+			log.Printf("shutdown metric provider: %v", err)
+		}
+	}()
+
+	jobsProcessed.Inc()
+}
+```
+
+## Notes
+
+- If your application uses the default Prometheus registry, you can omit
+  `bridgeprometheus.WithGatherer(reg)` and `NewMetricProducer` will read from
+  `prometheus.DefaultGatherer`.
+- `Shutdown` flushes pending metric data. For short-lived jobs, make sure you
+  shut the provider down before exit.
+- If you also use native OpenTelemetry metrics, attach them to the same
+  `MeterProvider` so both pipelines are exported together.
+
+## Related documentation
+
+- Prometheus bridge package:
+  <https://pkg.go.dev/go.opentelemetry.io/contrib/bridges/prometheus>
+- OTLP metric HTTP exporter:
+  <https://pkg.go.dev/go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetrichttp>