support stdlib request.GetBody on metrics (#7931)

Based on https://github.com/open-telemetry/opentelemetry-go/pull/7794

This solves the exact same problem but on metrics. 

> traces export: Post "https://***/v1/metrics": http2: Transport: cannot
retry err [http2: Transport received Server's graceful shutdown GOAWAY]
after Request.Body was written; define Request.GetBody to avoid this
error

---------

Co-authored-by: Damien Mathieu <42@dmathieu.com>

CHANGELOG.md

@@ -14,6 +14,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
   The package contains semantic conventions from the `v1.40.0` version of the OpenTelemetry Semantic Conventions.
   See the [migration documentation](./semconv/v1.40.0/MIGRATION.md) for information on how to upgrade from `go.opentelemetry.io/otel/semconv/v1.39.0`. (#7985)
 
+### Fixed
+
+- Fix missing `request.GetBody` in `go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetrichttp` to correctly handle HTTP2 GOAWAY frame. (#7931)
+
 ### Removed
 
 - Drop support for [Go 1.24]. (#7984)

exporters/otlp/otlpmetric/otlpmetrichttp/client.go

@@ -243,6 +243,7 @@ func (c *client) newRequest(ctx context.Context, body []byte) (request, error) {
 	case NoCompression:
 		r.ContentLength = int64(len(body))
 		req.bodyReader = bodyReader(body)
+		req.GetBody = bodyReaderErr(body)
 	case GzipCompression:
 		// Ensure the content length is not used.
 		r.ContentLength = -1
@@ -263,6 +264,7 @@ func (c *client) newRequest(ctx context.Context, body []byte) (request, error) {
 		}
 
 		req.bodyReader = bodyReader(b.Bytes())
+		req.GetBody = bodyReaderErr(body)
 	}
 
 	return req, nil
@@ -275,6 +277,13 @@ func bodyReader(buf []byte) func() io.ReadCloser {
 	}
 }
 
+// bodyReaderErr returns a closure returning a new reader for buf.
+func bodyReaderErr(buf []byte) func() (io.ReadCloser, error) {
+	return func() (io.ReadCloser, error) {
+		return io.NopCloser(bytes.NewReader(buf)), nil
+	}
+}
+
 // request wraps an http.Request with a resettable body reader.
 type request struct {
 	*http.Request

exporters/otlp/otlpmetric/otlpmetrichttp/client_test.go

@@ -8,9 +8,12 @@ import (
 	"crypto/tls"
 	"errors"
 	"fmt"
+	"io"
 	"net/http"
+	"net/http/httptest"
 	"net/url"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
@@ -324,3 +327,55 @@ func TestConfig(t *testing.T) {
 		assert.NoError(t, exCtx.Err())
 	})
 }
+
+func TestGetBodyCalledOnRedirect(t *testing.T) {
+	// Test that req.GetBody is set correctly, allowing the HTTP transport
+	// to re-send the body on 307 redirects.
+	var mu sync.Mutex
+	var requestBodies [][]byte
+
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, err := io.ReadAll(r.Body)
+		if err != nil {
+			w.WriteHeader(http.StatusInternalServerError)
+			return
+		}
+
+		mu.Lock()
+		requestBodies = append(requestBodies, body)
+		isFirstRequest := len(requestBodies) == 1
+		mu.Unlock()
+
+		if isFirstRequest {
+			w.Header().Set("Location", "/v1/metrics/final")
+			w.WriteHeader(http.StatusTemporaryRedirect)
+			return
+		}
+
+		w.Header().Set("Content-Type", "application/x-protobuf")
+		w.WriteHeader(http.StatusOK)
+	})
+
+	server := httptest.NewServer(handler)
+	defer server.Close()
+
+	opts := []Option{WithEndpoint(server.Listener.Addr().String()), WithInsecure()}
+	cfg := oconf.NewHTTPConfig(asHTTPOptions(opts)...)
+	client, err := newClient(cfg)
+	require.NoError(t, err)
+
+	exporter, err := newExporter(client, cfg)
+	require.NoError(t, err)
+	ctx := t.Context()
+	defer func() { _ = exporter.Shutdown(ctx) }()
+
+	err = exporter.Export(ctx, &metricdata.ResourceMetrics{})
+	require.NoError(t, err)
+
+	mu.Lock()
+	defer mu.Unlock()
+
+	require.Len(t, requestBodies, 2, "expected 2 requests (original + redirect)")
+	assert.NotEmpty(t, requestBodies[0], "original request body should not be empty")
+	assert.Equal(t, requestBodies[0], requestBodies[1], "redirect body should match original")
+}