thrown exceptions are not reported to the HTTP counter on ring collector

[erez-rabih]

I'm using ring/compojure to run my JSON api server
I use the wrap-instrumentation ring collector as a middleware to report prometheus metrics

My problem is that the current collector does not report thrown exceptions as 5XX status class to the http_requests_total metric
I know it increments http_exceptions_total but I still find it strange that if I those exceptions are not counted anywhere in the requests total or the latency histogram
I guess my questions are:

1. Have I missed anything or is this the behavior the ring collector works with?
2. If this is the current behavior I would like to suggest a new parameter to the options map, let's call it :exception-response which is a ring map - for example {:status 500 :body "something bad happened"}
   we can then use the record-metrics! on the ring map in case we have exception here to have both HTTP response code and latency monitored in case of a thrown exception

would be happy to get your opinion on this

[erez-rabih]

I prepared an example PR #48 that implements this logic