feat: connection pool metrics in admin server

src/PostgREST/Query.hs

@@ -43,6 +43,7 @@ import PostgREST.Config                  (AppConfig (..),
 import PostgREST.Config.PgVersion        (PgVersion (..))
 import PostgREST.Error                   (Error)
 import PostgREST.MediaType               (MediaType (..))
+import PostgREST.Observation             (Observation (..))
 import PostgREST.Plan                    (ActionPlan (..),
                                           CallReadPlan (..),
                                           CrudPlan (..),
@@ -77,10 +78,16 @@ data QueryResult
 runQuery :: AppState.AppState -> AppConfig -> AuthResult -> ApiRequest -> ActionPlan -> SchemaCache -> PgVersion -> Bool -> ExceptT Error IO QueryResult
 runQuery _ _ _ _ (NoDb x) _ _ _ = pure $ NoDbResult x
 runQuery appState config AuthResult{..} apiReq (Db plan) sCache pgVer authenticated = do
+  let observer = AppState.getObserver appState
+
+  lift $ observer PoolRequest
+
   dbResp <- lift $ do
     let transaction = if prepared then SQL.transaction else SQL.unpreparedTransaction
     AppState.usePool appState (transaction isoLvl txMode $ runExceptT dbHandler)
 
+  lift $ observer PoolRequestFullfilled
+
   resp <-
     liftEither . mapLeft Error.PgErr $
       mapLeft (Error.PgError authenticated) dbResp