chore(docs): update the metrics documentation (#1732), r=@vbudhram
chore(docs): update the metrics documentation (#1732), r=@vbudhram


mozilla/fxa-auth-server,eoger/fxa-auth-server,eoger/fxa-auth-server,eoger/fxa-auth-server,mozilla/fxa-auth-server,eoger/fxa-auth-server,mozilla/fxa-auth-server,mozilla/fxa-auth-server