* add $upstream_cache_status
* add '-$connection_requests' after $connection
* enable subrequest logging
$connection_requests makes it much easier to see connection reuse in the
logs and also helps to understand subrequests.
This switches to a fully custom log format instead of using a variant of
the standard combined format since we don't use any tools requiring the
logs to be a standard format. This provides a cleaner format, allows us
to freely add new fields and gets rid of legacy/redundant fields.
The redundant timestamp already provided as the syslog timestamp is
dropped along with the legacy identd field always set to a dash.
This adds the connection serial number for identifying requests coming
from the same connection. TLS version is added as a replacement for our
previous addition of the URI scheme. This also adds the total request
length and total bytes sent to the client instead of only the body bytes
sent.
This only improves performance for the initial page load by sending
resources that are almost always needed before the client receives the
preload headers and fetches them. It can degrade performance in some
edge cases such as clients with web fonts disabled or if the session
cookie is cleared without the cache being cleared. Clients can cancel
the push transfers once they start receiving them, but it's wasteful.
Safari and Firefox still support this feature but are likely to follow
the lead of Chromium and drop support for it. Few websites are going to
bother with it without Chromium support and usage is already dropping.
