Add prometheus metrics support

[arianvp]

I was able to add prometheus metrics supports with this super small snippet:

import Data.Proxy (Proxy)
import Data.Text (Text)
import qualified Data.Text as Text
import Network.Wai (Application, Request)
import Network.Wai.Middleware.Prometheus (instrumentHandlerValue)
import Servant.Ekg
  ( APIEndpoint (APIEndpoint),
    HasEndpoint (getEndpoint),
  )

-- | A middleware that records latency metrics for all endpoints in 'api'
instrumentServantHandlers :: HasEndpoint api => Proxy api -> Application -> Application
instrumentServantHandlers proxy app = do
  instrumentHandlerValue (makeLabel proxy) app

makeLabel :: HasEndpoint api => Proxy api -> Request -> Text
makeLabel proxy req = case getEndpoint proxy req of
  Nothing -> "<unknown>"
  Just (APIEndpoint pathParts _method) -> "/" <> Text.intercalate "/" pathParts

type GetMetrics = "metrics" :> Get '[PlainText] Text
handleGetMetrics :: Handler Text
handleGetMetrics = do
  Encoding.decodeUtf8With Encoding.lenientDecode <$> Prometheus.exportMetricsAsText

It only depends on the HasEndpoint typeclass. I'm wondering if maybe we should move HasEndpoint into the servant core package to make Prometheus support possible without people having to pull in the ekg dependency? cc @haskell-servant/maintainers

IDK if servant-prometheus deserves its only library or should just be a cookbook with the above code.

[arianvp]

Thoughts @haskell-servant/maintainers ?

edit: Oh I already mentioned the group. Sorry for the double ping. that was not my intention

[tchoutri]

eh I guess 26 days is an acceptable period for re-ping ;)

[tchoutri]

@arianvp here are my thoughts:

1. This overall very good
2. Yes we must avoid depending on the whole of ekg (especially snap) when providing this prometheus implementation.
3. Does it need a separate library? Eeeh I'm not convinced, we're not in the JavaScript world and this snippet clearly doesn't cross the Fairbairn threshold. We have two options:
   1. put it in a Cookbook for now, if it grows a lot we can think about a new library.
   2. put it in servant-ekg, and if it grows too much we can think about a new library.

[alpmestan]

Could your functions be merged into servant-ekg?

[arianvp]

They could but I'd prefer not to pull in ekg when using prometheus as my functions don't use anything ekg related. Just te HasEndpoint typeclass https://hackage.haskell.org/package/servant-ekg-0.3.1/docs/Servant-Ekg.html#t:HasEndpoint

[tchoutri]

Ah, if HasEndpoint is the only one, maybe we could upstream it outside of servant-ekg.

[worm2fed]

You might be interested
https://github.com/worm2fed/servant-prometheus