code: private UnaryCallable<String, List<KeyOffset>> createSampleRowKeysCallable() {
      UnaryCallable<SampleRowKeysRequest, List<SampleRowKeysResponse>> spoolable =
          stub.sampleRowKeysCallable().all();
  
      UnaryCallable<SampleRowKeysRequest, List<SampleRowKeysResponse>> retryable =
          Callables.retrying(spoolable, settings.sampleRowKeysSettings(), clientContext);
  
      return createUserFacingUnaryCallable(
          "SampleRowKeys", new SampleRowKeysCallable(retryable, requestContext));
    }
docs: Creates a callable chain to handle SampleRowKeys RPcs. The chain will:
  
  <ul>
  <li>Convert a table id to a {@link com.google.bigtable.v2.SampleRowKeysRequest}.
  <li>Dispatch the request to the GAPIC's {@link BigtableStub#sampleRowKeysCallable()}.
  <li>Spool responses into a list.
  <li>Retry on failure.
  <li>Convert the responses into {@link KeyOffset}s.
  </ul>