feat(metrics): track templates used in sent email
feat(metrics): track templates used in sent email

Fixes #130

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