Audit and update Metrics

[jrconlin]

We currently keep a list of Autopush metrics at:

https://docs.google.com/spreadsheets/d/18RL-Brz-Q-jR-iClNPCwDMVPYS0P2CvkHSJ2WZK-ceA/edit#gid=0

I don't know if we have similar spreadsheets for durable sync metrics, but we should.

We need to audit these to determine what metrics are useful both to dev and product, how they're being consumed, and what dashboards they need to drive.

(We may wish to coordinate some of these with other teams like Application Services for sync and client telemetry for Push. This was driven as part of R F Kelly's Making Sense of Send Tab Metrics

[jrconlin]

worksheet of metrics: https://docs.google.com/document/d/1FxPTh_OFYRY3EQnVxB0vRTuxiWS9yvCennhzjH-dZzE/edit#heading=h.iyv14dl47efm

[jrconlin]

Closing for now in lieu of other issues.
e.g. mozilla-services/syncstorage-rs#796
mozilla-services/pushbox#72
mozilla-services/megaphone#83
etc.