add user syslog writers to app (#970)

* add user syslog writers to app

users may specify a syslog url[s] on apps now and all functions under that app
will spew their logs out to it. the docs have more information around details
there, please review those (swagger and operating/logging.md), tried to
implement to spec in some parts and improve others, open to feedback on
format though, lots of liberty there.

design decision wise, I am looking to the future and ignoring cold containers.
the overhead of the connections there will not be worth it, so this feature
only works for hot functions, since we're killing cold anyway (even if a user
can just straight up exit a hot container).

syslog connections will be opened against a container when it starts up, and
then the call id that is logged gets swapped out for each call that goes
through the container, this cuts down on the cost of opening/closing
connections significantly. there are buffers to accumulate logs until we get a
`\n` to actually write a syslog line, and a buffer to save some bytes when
we're writing the syslog formatting as well. underneath writers re-use the
line writer in certain scenarios (swapper). we could likely improve the ease
of setting this up, but opening the syslog conns against a container seems
worth it, and is a different path than the other func loggers that we create
when we make a call object. the Close() stuff is a little tricky, not sure how
to make it easier and have the ^ benefits, open to idears.

this does add another vector of 'limits' to consider for more strict service
operators. one being how many syslog urls can a user add to an app (infinite,
atm) and the other being on the order of number of containers per host we
could run out of connections in certain scenarios. there may be some utility
in having multiple syslog sinks to send to, it could help with debugging at
times to send to another destination or if a user is a client w/ someone and
both want the function logs, e.g. (have used this for that in the past,
specifically).

this also doesn't work behind a proxy, which is something i'm open to fixing,
but afaict will require a 3rd party dependency (we can pretty much steal what
docker does). this is mostly of utility for those of us that work behind a
proxy all the time, not really for end users.

there are some unit tests. integration tests for this don't sound very fun to
maintain. I did test against papertrail with each protocol and it works (and
even times out if you're behind a proxy!).

closes #337

* add trace to syslog dial

docs/operating/logging.md

@@ -24,9 +24,30 @@ Note the easily searchable `call_id=x` format.
 call_id=477949e2-922c-5da9-8633-0b2887b79f6e
 ```
 
-## Metrics
+## Remote syslog for functions
 
-Metrics are emitted via the logs.
+You may add a syslog url to any function application and all functions that
+exist under that application will ship all of their logs to it. You may
+provide a comma separated list, if desired. Currently, we support `tcp`,
+`udp`, and `tls`, and this will not work if behind a proxy [yet?] (this is my
+life now). This feature only works for 'hot' functions.
 
-See [Metrics](metrics.md) doc for more information.
+An example syslog url is:
 
+```
+tls://logs.papertrailapp.com:1
+```
+
+We log in a syslog format, with some variables added in logfmt format. If you
+find logfmt format offensive, please open an issue and we will consider adding
+more formats (or open a PR that does it, with tests, and you will receive 1
+free cookie along with the feature you want). The logs from the functions
+themselves are not formatted, only our pre-amble, thus, if you'd like a fully
+logfmt line, you must use a logfmt logger to log from your function.
+
+* All log lines are sent as level error w/ the current time and `fn` as hostname.
+* call_id, func_name, and app_id will prefix every log line.
+
+```
+<11>2 1982-06-25T12:00:00Z fn - - - - call_id=12345 func_name=yo/yo app_id=54321 this is your log line
+```