[samplecode] Implement rpc default method sample code

[summer-ji-eng]

RPC default method has 3 types sample code for:

1. paged (gapic-generator sample code)

  * try (LibraryServiceClient libraryServiceClient = LibraryServiceClient.create()) {
  *   ListShelvesRequest request =
  *       ListShelvesRequest.newBuilder()
  *           .setPageSize(883849137)
  *           .setPageToken("pageToken873572522")
  *           .build();
  *   for (Shelf element : libraryServiceClient.listShelves(request).iterateAll()) {
  *     // doThingsWith(element);
  *   }
  * }

2. lro (gapic-generator sample code)

   * try (CloudRedisClient cloudRedisClient = CloudRedisClient.create()) {
   *   UpgradeInstanceRequest request =
   *       UpgradeInstanceRequest.newBuilder()
   *           .setName(InstanceName.of("[PROJECT]", "[LOCATION]", "[INSTANCE]").toString())
   *           .setRedisVersion("redisVersion-1972584739")
   *           .build();
   *   Instance response = cloudRedisClient.upgradeInstanceAsync(request).get();
   * }

or if method return void, the last line is cloudRedisClient.upgradeInstanceAsync(request).get();
3. pure unary (gapic-generator sample code)

   * try (MetricsClient metricsClient = MetricsClient.create()) {
   *   CreateLogMetricRequest request =
   *       CreateLogMetricRequest.newBuilder()
   *           .setParent(LogMetricName.of("[PROJECT]", "[METRIC]").toString())
   *           .setMetric(LogMetric.newBuilder().build())
   *           .build();
   *   LogMetric response = metricsClient.createLogMetric(request);
   * }

Or if the method return void, the last line is metricsClient.createLogMetric(request);

[miraleung]

Similarly, this could probably reuse composeUnaryRpcMethodSampleCode and composeUnaryLroRpcMethodSampleCode.

[summer-ji-eng]

Hi @miraleung, thanks for the valuable suggestion.

Make composeUnaryRpcMethodSampleCode, composeUnaryPagedRpcMethodSampleCode, and composeUnaryLroRpcMethodSampleCode re-usable for default method.

Update the unit test based on the new approach.

Open to discuss a better approach to re-use the utility methods. 🙏

[miraleung]

This approach is much better, thanks! LGTM with last comments addressed.

[miraleung]

Do we need any extra handling for streaming methods?

[summer-ji-eng]

No need to handle streaming methods here in composeRpcMethodHeaderSampleCode.
The caller is createMethodVariants only pass in the non-stream method in:

if (method.stream().equals(Stream.NONE)) {
        javaMethods.addAll(
            createMethodVariants(method, clientName, messageTypes, types, resourceNames));
        javaMethods.add(
            createMethodDefaultMethod(method, clientName, messageTypes, types, resourceNames));
      }

[summer-ji-eng]

Thanks for reviewing @miraleung. Merge into samplecode-master now.