fix(docs): overhaul the metrics events documentation
fix(docs): overhaul the metrics events documentation

https://github.com/mozilla/fxa-auth-server/pull/1901

r=vbudhram
eoger/fxa-auth-server,eoger/fxa-auth-server,mozilla/fxa-auth-server,eoger/fxa-auth-server,mozilla/fxa-auth-server,eoger/fxa-auth-server,mozilla/fxa-auth-server,mozilla/fxa-auth-server