feat: include client version in user agent header (#747)

* chore: add DirectPath fallback integration test

* feat: include User agent

google-cloud-spanner/src/main/java/com/google/cloud/spanner/spi/v1/GapicSpannerRpc.java

@@ -35,6 +35,7 @@
 import com.google.api.gax.rpc.ApiCallContext;
 import com.google.api.gax.rpc.ApiClientHeaderProvider;
 import com.google.api.gax.rpc.ApiException;
+import com.google.api.gax.rpc.FixedHeaderProvider;
 import com.google.api.gax.rpc.HeaderProvider;
 import com.google.api.gax.rpc.InstantiatingWatchdogProvider;
 import com.google.api.gax.rpc.OperationCallable;
@@ -70,6 +71,7 @@
 import com.google.common.base.MoreObjects;
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.common.util.concurrent.RateLimiter;
 import com.google.common.util.concurrent.ThreadFactoryBuilder;
 import com.google.iam.v1.GetIamPolicyRequest;
@@ -232,6 +234,8 @@ private void awaitTermination() throws InterruptedException {
   private static final int DEFAULT_TIMEOUT_SECONDS = 30 * 60;
   private static final int DEFAULT_PERIOD_SECONDS = 10;
   private static final int GRPC_KEEPALIVE_SECONDS = 2 * 60;
+  private static final String USER_AGENT_KEY = "user-agent";
+  private static final String CLIENT_LIBRARY_LANGUAGE = "spanner-java";
 
   // TODO(weiranf): Remove this temporary endpoint once DirectPath goes to public beta.
   private static final String DIRECT_PATH_ENDPOINT = "aa423245250f2bbf.sandbox.googleapis.com:443";
@@ -297,9 +301,20 @@ public GapicSpannerRpc(final SpannerOptions options) {
             .build();
 
     HeaderProvider mergedHeaderProvider = options.getMergedHeaderProvider(internalHeaderProvider);
+    Map<String, String> headersWithUserAgent =
+        ImmutableMap.<String, String>builder()
+            .put(
+                USER_AGENT_KEY,
+                CLIENT_LIBRARY_LANGUAGE
+                    + "/"
+                    + GaxProperties.getLibraryVersion(GapicSpannerRpc.class))
+            .putAll(mergedHeaderProvider.getHeaders())
+            .build();
+    final HeaderProvider headerProviderWithUserAgent =
+        FixedHeaderProvider.create(headersWithUserAgent);
     this.metadataProvider =
         SpannerMetadataProvider.create(
-            mergedHeaderProvider.getHeaders(),
+            headerProviderWithUserAgent.getHeaders(),
             internalHeaderProviderBuilder.getResourceHeaderKey());
     this.callCredentialsProvider = options.getCallCredentialsProvider();
     this.compressorName = options.getCompressorName();
@@ -338,7 +353,7 @@ public GapicSpannerRpc(final SpannerOptions options) {
                             options.getInterceptorProvider(),
                             SpannerInterceptorProvider.createDefault()))
                     .withEncoding(compressorName))
-            .setHeaderProvider(mergedHeaderProvider);
+            .setHeaderProvider(headerProviderWithUserAgent);
 
     // TODO(weiranf): Set to true by default once DirectPath goes to public beta.
     if (shouldAttemptDirectPath()) {
@@ -478,8 +493,10 @@ private static void checkEmulatorConnection(
         throw SpannerExceptionFactory.newSpannerException(
             ErrorCode.UNAVAILABLE,
             String.format(
-                "The environment variable SPANNER_EMULATOR_HOST has been set to %s, but no running emulator could be found at that address.\n"
-                    + "Did you forget to start the emulator, or to unset the environment variable?",
+                "The environment variable SPANNER_EMULATOR_HOST has been set to %s, but no running"
+                    + " emulator could be found at that address.\n"
+                    + "Did you forget to start the emulator, or to unset the environment"
+                    + " variable?",
                 emulatorHost));
       }
     }