add: log pool flushes

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

src/PostgREST/Observation.hs

@@ -64,6 +64,7 @@ data Observation
   | HasqlPoolObs SQL.Observation
   | PoolRequest
   | PoolRequestFullfilled
+  | PoolFlushed
   | JwtCacheLookup Bool
   | JwtCacheEviction
   | TerminationUnixSignalObs Text
@@ -161,6 +162,8 @@ observationMessage = \case
     "Trying to borrow a connection from pool"
   PoolRequestFullfilled ->
     "Borrowed a connection from the pool"
+  PoolFlushed ->
+    "Database connection pool flushed"
   JwtCacheLookup _ ->
     "Looked up a JWT in JWT cache"
   JwtCacheEviction ->