chore: expose the user set executor on TransportProvider (#13557)

This allows service clients to reuse that executor for other transport
related things in addition to gax

Author: igorbernstein2

java-bigtable/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/gaxx/grpc/BigtableTransportChannelProvider.java

@@ -74,6 +74,12 @@ public boolean needsExecutor() {
     return delegate.needsExecutor();
   }
 
+  @Nullable
+  @Override
+  public Executor getExecutor() {
+    return delegate.getExecutor();
+  }
+
   @Override
   @Deprecated
   public BigtableTransportChannelProvider withExecutor(ScheduledExecutorService executor) {

sdk-platform-java/gax-java/gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java

@@ -264,6 +264,12 @@ public boolean needsExecutor() {
     return executor == null;
   }
 
+  @Nullable
+  @Override
+  public Executor getExecutor() {
+    return executor;
+  }
+
   @Deprecated
   @Override
   public TransportChannelProvider withExecutor(ScheduledExecutorService executor) {

sdk-platform-java/gax-java/gax-httpjson/src/main/java/com/google/api/gax/httpjson/InstantiatingHttpJsonChannelProvider.java

@@ -105,6 +105,12 @@ public boolean needsExecutor() {
     return executor == null;
   }
 
+  @Nullable
+  @Override
+  public Executor getExecutor() {
+    return executor;
+  }
+
   @Deprecated
   @Override
   public TransportChannelProvider withExecutor(ScheduledExecutorService executor) {

sdk-platform-java/gax-java/gax/src/main/java/com/google/api/gax/rpc/TransportChannelProvider.java

@@ -36,6 +36,7 @@
 import java.util.Map;
 import java.util.concurrent.Executor;
 import java.util.concurrent.ScheduledExecutorService;
+import javax.annotation.Nullable;
 
 /**
  * Provides an interface to either build a TransportChannel or provide a fixed TransportChannel that
@@ -70,6 +71,15 @@ public interface TransportChannelProvider {
   @Deprecated
   boolean needsExecutor();
 
+  /**
+   * @return the user provided executor. This can be null if the user didn't override the executor
+   *     and/or the TransportProvider is using its internal executor.
+   */
+  @Nullable
+  default Executor getExecutor() {
+    return null;
+  }
+
   /** Sets the executor to use when constructing a new {@link TransportChannel}. */
   TransportChannelProvider withExecutor(Executor executor);