Insert custom headers via HeaderInterceptor.

src/main/java/com/google/api/gax/grpc/HeaderInterceptor.java

@@ -0,0 +1,37 @@
+package com.google.api.gax.grpc;
+
+import io.grpc.CallOptions;
+import io.grpc.Channel;
+import io.grpc.ClientCall;
+import io.grpc.ClientInterceptor;
+import io.grpc.ForwardingClientCall.SimpleForwardingClientCall;
+import io.grpc.Metadata;
+import io.grpc.MethodDescriptor;
+
+/**
+ * An intercepter to handle custom header.
+ */
+public class HeaderInterceptor implements ClientInterceptor {
+  private static final Metadata.Key<String> HEADER_KEY =
+      Metadata.Key.of("x-google-apis-agent", Metadata.ASCII_STRING_MARSHALLER);
+  private final String header;
+
+  public HeaderInterceptor(String header) {
+    this.header = header;
+  }
+
+  @Override
+  public <ReqT, RespT> ClientCall<ReqT, RespT> interceptCall(
+      MethodDescriptor<ReqT, RespT> method,
+      CallOptions callOptions,
+      Channel next) {
+    ClientCall<ReqT, RespT> call = next.newCall(method, callOptions);
+    return new SimpleForwardingClientCall<ReqT, RespT>(call) {
+      @Override
+      public void start(ClientCall.Listener<RespT> responseListener, Metadata headers) {
+        headers.put(HEADER_KEY, header);
+        super.start(responseListener, headers);
+      }
+    };
+  }
+}

src/main/java/com/google/api/gax/grpc/ServiceApiSettings.java

@@ -31,6 +31,8 @@
  * and should not be used in production.
  */
 public class ServiceApiSettings {
+  private String serviceGeneratorName;
+  private String serviceGeneratorVersion;
   private ChannelProvider channelProvider;
   private ExecutorProvider executorProvider;
   private final ImmutableList<? extends ApiCallSettings> allMethods;
@@ -40,6 +42,12 @@ public class ServiceApiSettings {
    */
   public static final int DEFAULT_EXECUTOR_THREADS = 4;
 
+  /**
+   * Default name and version of the service generator.
+   */
+  private static final String DEFAULT_GENERATOR_NAME = "gapic";
+  private static final String DEFAULT_GENERATOR_VERSION = "0.0.0";
+
   /**
    * Constructs an instance of ServiceApiSettings.
    */
@@ -99,13 +107,26 @@ public ManagedChannel getChannel(Executor executor) throws IOException {
 
         List<ClientInterceptor> interceptors = Lists.newArrayList();
         interceptors.add(new ClientAuthInterceptor(settings.getCredentials(), executor));
+        interceptors.add(new HeaderInterceptor(serviceHeader()));
 
         channel = NettyChannelBuilder.forAddress(settings.getServiceAddress(), settings.getPort())
             .negotiationType(NegotiationType.TLS)
             .intercept(interceptors)
             .build();
         return channel;
       }
+
+      private String serviceHeader() {
+        // GAX version only works when the package is invoked as a jar.
+        String gaxVersion = ChannelProvider.class.getPackage().getImplementationVersion();
+        String javaVersion = Runtime.class.getPackage().getImplementationVersion();
+        String generatorName = serviceGeneratorVersion.isEmpty() ?
+            DEFAULT_GENERATOR_NAME : serviceGeneratorName;
+        String generatorVersion = serviceGeneratorVersion.isEmpty() ?
+            DEFAULT_GENERATOR_VERSION : serviceGeneratorVersion;
+        return String.format("gax-%s/java-%s/%s-%s",
+            gaxVersion, javaVersion, generatorName, generatorVersion);
+      }
     };
     return this;
   }
@@ -190,4 +211,12 @@ public ServiceApiSettings setRetryParamsOnAllMethods(RetryParams retryParams) {
     }
     return this;
   }
+
+  /**
+   * Sets the generator name and version for the GRPC custom header.
+   */
+  public void setGeneratorHeader(String name, String version) {
+    this.serviceGeneratorName = name;
+    this.serviceGeneratorVersion = version;
+  }
 }

src/test/java/com/google/api/gax/grpc/HeaderInterceptorTest.java

@@ -0,0 +1,69 @@
+package com.google.api.gax.grpc;
+
+import static org.junit.Assert.assertEquals;
+import static org.mockito.Matchers.any;
+import static org.mockito.Matchers.same;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+import org.mockito.ArgumentCaptor;
+import org.mockito.Mock;
+import org.mockito.Mockito;
+import org.mockito.MockitoAnnotations;
+
+import io.grpc.CallOptions;
+import io.grpc.Channel;
+import io.grpc.ClientCall;
+import io.grpc.ClientInterceptors;
+import io.grpc.Metadata;
+import io.grpc.MethodDescriptor;
+
+/**
+ * Tests for {@link HeaderInterceptor}.
+ */
+@RunWith(JUnit4.class)
+public class HeaderInterceptorTest {
+
+  @Mock
+  private Channel channel;
+
+  @Mock
+  private ClientCall<String, Integer> call;
+
+  @Mock
+  private MethodDescriptor<String, Integer> method;
+
+  /**
+   * Sets up mocks.
+   */
+  @Before public void setUp() {
+    MockitoAnnotations.initMocks(this);
+    when(channel.newCall(
+        Mockito.<MethodDescriptor<String, Integer>>any(), any(CallOptions.class)))
+        .thenReturn(call);
+  }
+
+  @Test
+  public void testInterceptor() {
+    final Metadata.Key<String> headerKey =
+        Metadata.Key.of("x-google-apis-agent", Metadata.ASCII_STRING_MARSHALLER);
+    String data = "abcd";
+    HeaderInterceptor interceptor = new HeaderInterceptor(data);
+    Channel intercepted = ClientInterceptors.intercept(channel, interceptor);
+    @SuppressWarnings("unchecked")
+    ClientCall.Listener<Integer> listener = mock(ClientCall.Listener.class);
+    ClientCall<String, Integer> interceptedCall = intercepted.newCall(method, CallOptions.DEFAULT);
+    // start() on the intercepted call will eventually reach the call created by the real channel
+    interceptedCall.start(listener, new Metadata());
+    // The headers passed to the real channel call will contain the information inserted by the
+    // interceptor.
+    ArgumentCaptor<Metadata> captor = ArgumentCaptor.forClass(Metadata.class);
+    verify(call).start(same(listener), captor.capture());
+    assertEquals(data, captor.getValue().get(headerKey));
+  }
+}