feat: log bugbot usage

[ckerr]

As discussed in the Jul 1 meeting.

We want to be able to track how useful bugbot is to the project, and one meaningful proxy for that is how much it gets used.

Probably this should be done at the broker level by adding some persistent storage e.g. a database and logging information about issue / gist / login / os.platform / result

[MarshallOfSound]

@ckerr if you just care about raw counts we have a prometheus instance you can send metrics too. Or you can wire up a logdrain on heroku to our loki instance (which pipes into grafana).

If you actually need a DB to store structured info then ignore the above 😄