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/Observation.hs

@@ -49,6 +49,7 @@ data Observation
   | QueryRoleSettingsErrorObs SQL.UsageError
   | QueryErrorCodeHighObs SQL.UsageError
   | QueryPgVersionError SQL.UsageError
+  | PoolInit Int
   | PoolAcqTimeoutObs SQL.UsageError
   | HasqlPoolObs SQL.Observation
   | PoolRequest
@@ -118,6 +119,8 @@ observationMessage = \case
     "Failed reloading config: " <> err
   ConfigSucceededObs ->
      "Config reloaded"
+  PoolInit poolSize ->
+     "Connection Pool initialized with a maximum size of " <> show poolSize <> " connections"
   PoolAcqTimeoutObs usageErr ->
     jsonMessage usageErr
   HasqlPoolObs (SQL.ConnectionObservation uuid status) ->