Rate estimation for Pub/Sub Table

[damccorm]

Currently Pub/Sub table returns UNKNOWN as statistics

 

We can use Stackdriver monitoring (If it was available to the user) to get the rate for the pub/sub. (See https://docs.google.com/document/d/1vi1PBBu5IqSy-qZl1Gk-49CcANOpbNs1UAud6LnOaiY for sample code).

Imported from Jira BEAM-7897. Original Jira may contain additional context.
Reported by: riazela.