feat: Add opt-in flag and ClientInterceptor to propagate trace context for Spanner end to end tracing

google-cloud-spanner/pom.xml

@@ -255,6 +255,11 @@
       <groupId>io.opentelemetry</groupId>
       <artifactId>opentelemetry-context</artifactId>
     </dependency>
+    <dependency>
+      <groupId>io.opentelemetry.instrumentation</groupId>
+      <artifactId>opentelemetry-grpc-1.6</artifactId>
+      <version>2.1.0-alpha</version>
+    </dependency>
     <dependency>
       <groupId>com.google.auth</groupId>
       <artifactId>google-auth-library-oauth2-http</artifactId>

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

@@ -23,6 +23,7 @@
 import io.grpc.ClientInterceptor;
 import io.opentelemetry.api.GlobalOpenTelemetry;
 import io.opentelemetry.api.OpenTelemetry;
+import io.opentelemetry.instrumentation.grpc.v1_6.GrpcTelemetry;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.logging.Level;
@@ -51,6 +52,7 @@ public static SpannerInterceptorProvider createDefault(OpenTelemetry openTelemet
     defaultInterceptorList.add(
         new LoggingInterceptor(Logger.getLogger(GapicSpannerRpc.class.getName()), Level.FINER));
     defaultInterceptorList.add(new HeaderInterceptor(new SpannerRpcMetrics(openTelemetry)));
+    defaultInterceptorList.add(GrpcTelemetry.create(openTelemetry).newClientInterceptor());
     return new SpannerInterceptorProvider(ImmutableList.copyOf(defaultInterceptorList));
   }