fix(docs): Bring back the accidentally deleted client-metrics.md content. (#4593) r=vladikoff
fix(docs): Bring back the accidentally deleted client-metrics.md content. (#4593) r=vladikoff

A lot of content was accidentally deleted in #4370, this brings back the content.
shane-tomlinson/fxa-content-server,mozilla/fxa-content-server,mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,mozilla/fxa-content-server,shane-tomlinson/fxa-content-server