Embed partial ExecutedActionMetadata in ExecuteOperationMetadata (#238)

* Add support for auxiliary operation metadata

* Hint at using ExecutedActionMetadata

* Embed ExecutedActionMetadata directly

Author: roitk

build/bazel/remote/execution/v2/remote_execution.proto

@@ -1494,6 +1494,10 @@ message ExecuteOperationMetadata {
   // [ByteStream.Read][google.bytestream.ByteStream.Read] to stream the
   // standard error from the endpoint hosting streamed responses.
   string stderr_stream_name = 4;
+
+  // The client can read this field to view details about the ongoing
+  // execution.
+  ExecutedActionMetadata partial_execution_metadata = 5;
 }
 
 // A request message for