Make OtlpExporterOptions.HttpClientFactory work with GRPC protocol

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/OtlpExporterOptions.cs

@@ -157,6 +157,37 @@ public Uri Endpoint
         /// </summary>
         public BatchExportProcessorOptions<Activity> BatchExportProcessorOptions { get; set; }
 
+#if NETSTANDARD2_1 || NET6_0_OR_GREATER
+        /// <summary>
+        /// Gets or sets the factory function called to create the <see
+        /// cref="HttpClient"/> instance that will be used at runtime to
+        /// transmit telemetry over HTTP. The returned instance will be reused
+        /// for all export invocations.
+        /// </summary>
+        /// <remarks>
+        /// Notes:
+        /// <list type="bullet">
+        /// <item>The default behavior when using the <see
+        /// cref="OtlpTraceExporterHelperExtensions.AddOtlpExporter(TracerProviderBuilder,
+        /// Action{OtlpExporterOptions})"/> extension is if an <a
+        /// href="https://docs.microsoft.com/dotnet/api/system.net.http.ihttpclientfactory">IHttpClientFactory</a>
+        /// instance can be resolved through the application <see
+        /// cref="IServiceProvider"/> then an <see cref="HttpClient"/> will be
+        /// created through the factory with the name "OtlpTraceExporter"
+        /// otherwise an <see cref="HttpClient"/> will be instantiated
+        /// directly.</item>
+        /// <item>The default behavior when using the <see
+        /// cref="OtlpMetricExporterExtensions.AddOtlpExporter(MeterProviderBuilder,
+        /// Action{OtlpExporterOptions})"/> extension is if an <a
+        /// href="https://docs.microsoft.com/dotnet/api/system.net.http.ihttpclientfactory">IHttpClientFactory</a>
+        /// instance can be resolved through the application <see
+        /// cref="IServiceProvider"/> then an <see cref="HttpClient"/> will be
+        /// created through the factory with the name "OtlpMetricExporter"
+        /// otherwise an <see cref="HttpClient"/> will be instantiated
+        /// directly.</item>
+        /// </list>
+        /// </remarks>
+#else
         /// <summary>
         /// Gets or sets the factory function called to create the <see
         /// cref="HttpClient"/> instance that will be used at runtime to
@@ -188,6 +219,7 @@ public Uri Endpoint
         /// directly.</item>
         /// </list>
         /// </remarks>
+#endif
         public Func<HttpClient> HttpClientFactory { get; set; }
 
         /// <summary>

src/OpenTelemetry.Exporter.OpenTelemetryProtocol/OtlpExporterOptionsExtensions.cs

@@ -43,7 +43,11 @@ public static Channel CreateChannel(this OtlpExporterOptions options)
             }
 
 #if NETSTANDARD2_1 || NET6_0_OR_GREATER
-            return GrpcChannel.ForAddress(options.Endpoint);
+            return GrpcChannel.ForAddress(options.Endpoint, new GrpcChannelOptions
+            {
+                HttpClient = options.HttpClientFactory?.Invoke()
+                    ?? throw new InvalidOperationException("OtlpExporterOptions was missing HttpClientFactory or it returned null."),
+            });
 #else
             ChannelCredentials channelCredentials;
             if (options.Endpoint.Scheme == Uri.UriSchemeHttps)
@@ -130,7 +134,10 @@ public static THeaders GetHeaders<THeaders>(this OtlpExporterOptions options, Ac
         public static void TryEnableIHttpClientFactoryIntegration(this OtlpExporterOptions options, IServiceProvider serviceProvider, string httpClientName)
         {
             if (serviceProvider != null
+#if NETSTANDARD2_1 || NET6_0_OR_GREATER
+#else
                 && options.Protocol == OtlpExportProtocol.HttpProtobuf
+#endif
                 && options.HttpClientFactory == options.DefaultHttpClientFactory)
             {
                 options.HttpClientFactory = () =>