Two new metrics:
auth_db_connection_duration_seconds (histogram)
wraps the first DB call of post notifications. This includes waiting
to get a connection from the pool, and also making the actual request
to the db to retrieve the service and api keys. (i'm not sure there's
an easy way to separate these two things)
post_notification_json_parse_duration_seconds
wraps parsing the v2 post notifications json parsing and schema
validation. Shouldn't include any async code