otlploghttp: Add WithHTTPClient option (#6688)

Towards (for OTLP logs exporter):
- https://github.com/open-telemetry/opentelemetry-go/issues/4536
- https://github.com/open-telemetry/opentelemetry-go/issues/5129
- https://github.com/open-telemetry/opentelemetry-go/issues/2632


Per
https://github.com/open-telemetry/opentelemetry-go/pull/6362/files#r1978191352

Providing `WithHTTPClient` option allows easy interoperability e.g. with
https://pkg.go.dev/golang.org/x/oauth2/clientcredentials#Config.Client
and also see
https://github.com/open-telemetry/opentelemetry-go/issues/2632. The
options is also similar to:
https://pkg.go.dev/go.opentelemetry.io/otel/exporters/otlp/otlplog/otlploggrpc#WithGRPCConn

I favor this over
https://github.com/open-telemetry/opentelemetry-go/pull/6686

The option for OTLP span and metrics exporters will be added in separate
PRs.

CHANGELOG.md

@@ -15,6 +15,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
   The package contains semantic conventions from the `v1.31.0` version of the OpenTelemetry Semantic Conventions.
   See the [migration documentation](./semconv/v1.31.0/MIGRATION.md) for information on how to upgrade from `go.opentelemetry.io/otel/semconv/v1.30.0`(#6479)
 - Add `Recording`, `Scope`, and `Record` types in `go.opentelemetry.io/otel/log/logtest`. (#6507)
+- Add `WithHTTPClient` option to configure the `http.Client` used by `go.opentelemetry.io/otel/exporters/otlp/otlplog/otlploghttp`. (#6688)
 
 ### Removed
 

exporters/otlp/otlplog/otlploghttp/client.go

@@ -44,20 +44,23 @@ func newNoopClient() *client {
 
 // newHTTPClient creates a new HTTP log client.
 func newHTTPClient(cfg config) (*client, error) {
-	hc := &http.Client{
-		Transport: ourTransport,
-		Timeout:   cfg.timeout.Value,
-	}
-
-	if cfg.tlsCfg.Value != nil || cfg.proxy.Value != nil {
-		clonedTransport := ourTransport.Clone()
-		hc.Transport = clonedTransport
-
-		if cfg.tlsCfg.Value != nil {
-			clonedTransport.TLSClientConfig = cfg.tlsCfg.Value
+	hc := cfg.httpClient
+	if hc == nil {
+		hc = &http.Client{
+			Transport: ourTransport,
+			Timeout:   cfg.timeout.Value,
 		}
-		if cfg.proxy.Value != nil {
-			clonedTransport.Proxy = cfg.proxy.Value
+
+		if cfg.tlsCfg.Value != nil || cfg.proxy.Value != nil {
+			clonedTransport := ourTransport.Clone()
+			hc.Transport = clonedTransport
+
+			if cfg.tlsCfg.Value != nil {
+				clonedTransport.TLSClientConfig = cfg.tlsCfg.Value
+			}
+			if cfg.proxy.Value != nil {
+				clonedTransport.Proxy = cfg.proxy.Value
+			}
 		}
 	}
 

exporters/otlp/otlplog/otlploghttp/client_test.go

@@ -783,6 +783,28 @@ func TestConfig(t *testing.T) {
 		assert.Equal(t, []string{headerValueSetInProxy}, got[headerKeySetInProxy])
 	})
 
+	t.Run("WithHTTPClient", func(t *testing.T) {
+		headerKeySetInProxy := http.CanonicalHeaderKey("X-Using-Proxy")
+		headerValueSetInProxy := "true"
+		exp, coll := factoryFunc("", nil, WithHTTPClient(&http.Client{
+			Transport: &http.Transport{
+				Proxy: func(r *http.Request) (*url.URL, error) {
+					r.Header.Set(headerKeySetInProxy, headerValueSetInProxy)
+					return r.URL, nil
+				},
+			},
+		}))
+		ctx := context.Background()
+		t.Cleanup(func() { require.NoError(t, coll.Shutdown(ctx)) })
+		require.NoError(t, exp.Export(ctx, make([]log.Record, 1)))
+		// Ensure everything is flushed.
+		require.NoError(t, exp.Shutdown(ctx))
+
+		got := coll.Headers()
+		require.Contains(t, got, headerKeySetInProxy)
+		assert.Equal(t, []string{headerValueSetInProxy}, got[headerKeySetInProxy])
+	})
+
 	t.Run("non-retryable errors are propagated", func(t *testing.T) {
 		exporterErr := errors.New("missing required attribute aaaa")
 		rCh := make(chan exportResult, 1)

exporters/otlp/otlplog/otlploghttp/config.go

@@ -95,6 +95,7 @@ type config struct {
 	timeout     setting[time.Duration]
 	proxy       setting[HTTPTransportProxyFunc]
 	retryCfg    setting[retry.Config]
+	httpClient  *http.Client
 }
 
 func newConfig(options []Option) config {
@@ -344,6 +345,25 @@ func WithProxy(pf HTTPTransportProxyFunc) Option {
 	})
 }
 
+// WithHTTPClient sets the HTTP client to used by the exporter.
+//
+// This option will take precedence over [WithProxy], [WithTimeout],
+// [WithTLSClientConfig] options as well as OTEL_EXPORTER_OTLP_CERTIFICATE,
+// OTEL_EXPORTER_OTLP_LOGS_CERTIFICATE, OTEL_EXPORTER_OTLP_TIMEOUT,
+// OTEL_EXPORTER_OTLP_LOGS_TIMEOUT environment variables.
+//
+// Timeout and all other fields of the passed [http.Client] are left intact.
+//
+// Be aware that passing an HTTP client with transport like
+// [go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp.NewTransport] can
+// cause the client to be instrumented twice and cause infinite recursion.
+func WithHTTPClient(c *http.Client) Option {
+	return fnOpt(func(cfg config) config {
+		cfg.httpClient = c
+		return cfg
+	})
+}
+
 // setting is a configuration setting value.
 type setting[T any] struct {
 	Value T