This milestone follows after GCS+gRPC: Public Preview M1

At the end of this milestone the C++ plugin with be of sufficient quality for a public preview. Usually this means "ready for GA, except for waiting for any bug reports or feedback to come back." In this case this will include "the top 24 metadata operations" (sometimes "top 19", the actual count is not that relevant until the last few weeks of the deadline).

NOTE: this is subject to the service public preview release. We may complete the C++ client library plugin and then the service may be delayed for separate reasons.