gRPC's Paged Callables are not being Traced

[lqiu96]

From gRPC's CallableFactory:

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

Lines 129 to 139 in ba648ce

	public static <RequestT, ResponseT, PagedListResponseT>
	UnaryCallable<RequestT, PagedListResponseT> createPagedCallable(
	GrpcCallSettings<RequestT, ResponseT> grpcCallSettings,
	PagedCallSettings<RequestT, ResponseT, PagedListResponseT> pagedCallSettings,
	ClientContext clientContext) {
	UnaryCallable<RequestT, ResponseT> innerCallable =
	createBaseUnaryCallable(grpcCallSettings, pagedCallSettings, clientContext);
	UnaryCallable<RequestT, PagedListResponseT> pagedCallable =
	Callables.paged(innerCallable, pagedCallSettings);
	return pagedCallable.withDefaultCallContext(clientContext.getDefaultCallContext());
	}

Paged Callables do not have a TracedPagedCallable equivalent which implements tracing. It is possible that we can reused the TracedUnaryCallable, but that requires some investigation.

[blakeli0]

We should add a TracedPagedCallable similar to TracedUnaryCallable, and test with Showcase