Parse the user agent and log whether it seems like a known bot or a
human to NewRelic under the `user.bot` request attribute. This is so
that known bots can be filtered out of search traffic analytics. Bots
and search crawlers make up a significant portion of search traffic.
Log the Referer header, as well as the Sec-Fetch-* headers. These are
only sent by recent versions of Chrome; see https://www.w3.org/TR/fetch-metadata.
* Simplify code.
* Show backtraces for all users, not just builders.
* Show backtraces only for unexpected server errors (status 5xx), not
for normal client errors (status 4xx).
* Log expected errors at info level (reduce noise in production logs).
