add pgrst_pool_available gauge

src/PostgREST/Metrics.hs

@@ -6,27 +6,37 @@ module PostgREST.Metrics
   , metricsToText
   ) where
 
-import qualified Data.ByteString.Lazy as LBS
+import qualified Data.ByteString.Lazy   as LBS
+import qualified Hasql.Pool.Observation as SQL
 
 import qualified Prometheus as Prom
 
 import PostgREST.Observation
 
 import Protolude
 
-newtype MetricsState = MetricsState
-  { counterPoolTimeout          :: Prom.Counter
+data MetricsState = MetricsState
+  { poolTimeouts  :: Prom.Counter
+  , poolAvailable :: Prom.Gauge
   }
 
 init :: IO MetricsState
 init = do
-  counter <- Prom.register $ Prom.counter (Prom.Info "pgrst_pool_timeouts" "The number of connection timeouts in the pool")
-  pure $ MetricsState counter
+  counter <- Prom.register $ Prom.counter (Prom.Info "pgrst_pool_timeouts_total" "The number of connection timeouts in the pool")
+  gauge <- Prom.register $ Prom.gauge (Prom.Info "pgrst_pool_available" "The number of unused connections in the pool")
+  Prom.setGauge gauge 0
+  pure $ MetricsState counter gauge
 
 observationMetrics :: MetricsState -> ObservationHandler
-observationMetrics MetricsState{counterPoolTimeout} obs = case obs of
+observationMetrics MetricsState{poolTimeouts, poolAvailable} obs = case obs of
   (PoolAcqTimeoutObs _) -> do
-    Prom.incCounter counterPoolTimeout
+    Prom.incCounter poolTimeouts
+  (HasqlPoolObs (SQL.ConnectionObservation _ status)) -> case status of
+     SQL.ReadyForUseConnectionStatus  -> do
+      Prom.incGauge poolAvailable
+     SQL.InUseConnectionStatus        -> do
+      Prom.decGauge poolAvailable
+     _ -> pure ()
   _ ->
     pure ()