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

api/agent/call.go

@@ -103,6 +103,11 @@ func FromRequest(a Agent, app *models.App, path string, req *http.Request) CallO
 			return err
 		}
 
+		var syslogURL string
+		if app.SyslogURL != nil {
+			syslogURL = *app.SyslogURL
+		}
+
 		c.Call = &models.Call{
 			ID:    id,
 			Path:  route.Path,
@@ -123,6 +128,7 @@ func FromRequest(a Agent, app *models.App, path string, req *http.Request) CallO
 			URL:         reqURL(req),
 			Method:      req.Method,
 			AppID:       app.ID,
+			SyslogURL:   syslogURL,
 		}
 
 		c.req = req
@@ -258,9 +264,7 @@ func (a *agent) GetCall(opts ...CallOpt) (Call, error) {
 		logrus.Fields{"id": c.ID, "app_id": c.AppID, "route": c.Path})
 	c.req = c.req.WithContext(ctx)
 
-	// setup stderr logger separate (don't inherit ctx vars)
-	logger := logrus.WithFields(logrus.Fields{"user_log": true, "app_id": c.AppID, "path": c.Path, "image": c.Image, "call_id": c.ID})
-	c.stderr = setupLogger(logger, a.cfg.MaxLogSize)
+	c.stderr = setupLogger(ctx, a.cfg.MaxLogSize, c.Call)
 	if c.w == nil {
 		// send STDOUT to logs if no writer given (async...)
 		// TODO we could/should probably make this explicit to GetCall, ala 'WithLogger', but it's dupe code (who cares?)