add: log pool flushes

Emit a dedicated PoolFlushed observation when the DB pool is released during schema cache reload.

src/PostgREST/Logger.hs

@@ -110,6 +110,9 @@ observationLogger loggerState logLevel obs = case obs of
   o@PoolRequestFullfilled ->
     when (logLevel >= LogDebug) $ do
       logWithZTime loggerState $ observationMessage o
+  o@PoolFlushed ->
+    when (logLevel >= LogDebug) $ do
+      logWithZTime loggerState $ observationMessage o
   o@JwtCacheEviction ->
     when (logLevel >= LogDebug) $ do
       logWithZTime loggerState $ observationMessage o