feat: log pool maximum size

It's important for observability to have an historic trace of the pool
size. Currently we expose it on the metrics endpoint, but not all
deployments use it.

This logs the pool size after the successful connection log to make it
more visible:

<timestamp>: Connection Pool initialized with a maximum size of 4 connections

src/PostgREST/AppState.hs

@@ -132,6 +132,8 @@ init conf@AppConfig{configLogLevel, configDbPoolSize} = do
   metricsState <- Metrics.init configDbPoolSize
   let observer = liftA2 (>>) (Logger.observationLogger loggerState configLogLevel) (Metrics.observationMetrics metricsState)
 
+  observer $ AppStartObs prettyVersion
+
   pool <- initPool conf observer
   (sock, adminSock) <- initSockets conf
   state' <- initWithPool (sock, adminSock) pool conf loggerState metricsState observer
@@ -206,7 +208,7 @@ initSockets AppConfig{..} = do
   pure (sock, adminSock)
 
 initPool :: AppConfig -> ObservationHandler -> IO SQL.Pool
-initPool AppConfig{..} observer =
+initPool AppConfig{..} observer = do
   SQL.acquire $ SQL.settings
     [ SQL.size configDbPoolSize
     , SQL.acquisitionTimeout $ fromIntegral configDbPoolAcquisitionTimeout
@@ -391,6 +393,7 @@ retryingSchemaCacheLoad appState@AppState{stateObserver=observer, stateMainThrea
             observer $ ExitUnsupportedPgVersion actualPgVersion minimumPgVersion
             killThread mainThreadId
           observer $ DBConnectedObs $ pgvFullName actualPgVersion
+          observer $ PoolInit configDbPoolSize
           putPgVersion appState actualPgVersion
           return $ Just actualPgVersion