chore(logs): log an anonymous event when we see the DNT header
chore(logs): log an anonymous event when we see the DNT header

https://github.com/mozilla/fxa-content-server/pull/5760
r=vbudhram
mozilla/fxa-content-server,mozilla/fxa-content-server,mozilla/fxa-content-server,shane-tomlinson/fxa-content-server,shane-tomlinson/fxa-content-server,shane-tomlinson/fxa-content-server