Census contexts not propagated by in-process

[zhangkun83]

In-process transport currently disables Census altogether. However, we still need to propagate the Census StatsContext and tracing Span from the client to the server.

[zhangkun83]

/cc @ejona86

[zhangkun83]

This was fixed by #3517 and #3708.
#3517 brought tracing context propagation to in-process.
#3708 re-enabled CensusStatsModule for in-process.