ApiLogger plugin: dumps some information about API hits to aid in researching future...

ApiLogger plugin: dumps some information about API hits to aid in researching future HTTP-level cachability improvements.

Data are sent to the 'info' level of logging, like so:

  [lazarus.local:4812.86b23603 GET /mublog/api/statuses/friends_timeline.atom?since_id=1353]
       STATLOG action:apitimelinefriends method:GET ssl:no query:since_id cookie:no auth:yes
       ifmatch:no ifmod:no agent:Appcelerator Titanium/1.4.1 (iPhone/4.1; iPhone OS; en_US;)

Fields:
* action:  case-normalized name of the action class we're acting on
* method:  GET, POST, HEAD, etc
* ssl:     Are we on HTTPS? 'yes' or 'no'
* query:   Were we sent a query string? 'yes', 'no', or 'since_id' if the only parameter is a since_id
* cookie:  Were we sent any cookies? 'yes' or 'no'
* auth:    Were we sent an HTTP Authorization header? 'yes' or 'no'
* ifmatch: Were we sent an HTTP If-Match header for an ETag? 'yes' or 'no'
* ifmod:   Were we sent an HTTP If-Modified-Since header? 'yes' or 'no'
* agent:   User-agent string, to aid in figuring out what these things are

The most shared-cache-friendly requests will be non-SSL GET requests with no or very predictable
query parameters, no cookies, and no authorization headers. Private caching (eg within a supporting
user-agent) could still be friendly to SSL and auth'd GET requests.

We kind of expect that the most frequent hits from clients will be GETs for a few common timelines,
with auth headers, a since_id-only query, and no cookies. These should at least be amenable to
returning 304 matches for etags or last-modified headers with private caching, but it's very
possible that most clients won't actually think to save and send them. That would leave us expecting
to handle a lot of timeline since_id hits that return a valid API response with no notices.

At this point we don't expect to actually see if-match or if-modified-since a lot since most of our
API responses are marked as uncacheable; so even if we output them they're not getting sent back to
us.

Random subsampling can be enabled by setting the 'frequency' parameter smaller than 1.0:

  addPlugin('ApiLogger', array(
    'frequency' => 0.5 // Record 50% of API hits
  ));