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1. Only emit the receive-message metrics and logs if the callback has
   not already been invoked.
2. Record a fast-path-ready notification as a receive-message success.
3. Clarify receive-message histogram summaries to indicate that the
   metric records the result of opening the receive stream, i.e.,
   starting to receive messages, not a result for every message receipt.
4. Add metrics and logs to track send- and receive-message OAuth token
   fetches.

Manually verified logs and histograms.

Fixed: 1164009
Change-Id: I2a064cae444ec890971db40b15c93c713e32565c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2619628
Commit-Queue: Josh Nohle <nohle@chromium.org>
Reviewed-by: James Vecore <vecore@google.com>
Reviewed-by: Caitlin Fischer <caitlinfischer@google.com>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842617}