Prometheus: Exclude ResponseRaw responses from metrics (#2108)

This PR makes the `servantPrometheusMiddleware` ignore all `ResponseRaw` responses. The implementation of wai and prometheus lead to `ResponseRaw` being tracked with `statusCode` `500` irrespective if the handler was successful or not (see documentation of `wai-middleware-prometheus`'s `ignoreRawResponses` or note `[Raw Response]` in the codebase).

This would remove all metrics for these endpoints, however looking at `observeSeconds` of `wai-middleware-prometheus` suggests that the status is optional, so maybe reporting duration without status is possible.

This PR is a followup to #2081 which lead to `cannon`'s `"await-notifications"` endpoint erroneously reporting 500s metrics.

This will also affect future potential future endpoints in `spar` and `carghold` as they also use `servantPrometheusMiddleware`.

Author: smatting

changelog.d/5-internal/prometheus-ignore-raw-responses

@@ -0,0 +1 @@
+Prometheus: Ignore RawResponses (e.g. cannon's await responses) from metrics

libs/metrics-wai/src/Data/Metrics/Servant.hs

@@ -44,14 +44,17 @@ import Servant.Multipart
 
 -- | This does not catch errors, so it must be called outside of 'WU.catchErrors'.
 servantPrometheusMiddleware :: forall proxy api. (RoutesToPaths api) => proxy api -> Wai.Middleware
-servantPrometheusMiddleware _ = Promth.prometheus conf . Promth.instrumentHandlerValue promthNormalize
+servantPrometheusMiddleware _ = Promth.prometheus conf . instrument promthNormalize
   where
     promthNormalize :: Wai.Request -> Text
     promthNormalize req = pathInfo
       where
         mPathInfo = Metrics.treeLookup (routesToPaths @api) $ cs <$> Wai.pathInfo req
         pathInfo = cs $ fromMaybe "N/A" mPathInfo
 
+    -- See Note [Raw Response]
+    instrument = Promth.instrumentHandlerValueWithFilter Promth.ignoreRawResponses
+
 servantPlusWAIPrometheusMiddleware :: forall proxy api a m b. (RoutesToPaths api, Monad m) => Routes a m b -> proxy api -> Wai.Middleware
 servantPlusWAIPrometheusMiddleware routes _ = do
   Promth.prometheus conf . instrument (normalizeWaiRequestRoute paths)