refactor: get rid of redundant SchemaCacheSummaryObs

There is unnecessary coupling between observation messages and emited log entries. This causes schema loading logic to emit redundant events: SchemaCacheSummaryObs and SchemaCacheLoadedObs.

Logically - we want to emit a single event containing both summary and timing information. How it is logged is a different matter and should be decoupled.

This commit
* changes observationMessage function returning Text to observationMessages returning [Text] so that it is possible to return multiple (or zero) messages to log based on an observation event
* Removes SchemaCacheSummaryObs constructor from Observation type and adds summary text to SchemaCacheLoadedObs

src/PostgREST/Metrics.hs

@@ -64,7 +64,7 @@ observationMetrics MetricsState{..} obs = case obs of
     incGauge poolWaiting
   PoolRequestFullfilled ->
     decGauge poolWaiting
-  SchemaCacheLoadedObs resTime -> do
+  SchemaCacheLoadedObs resTime _ -> do
     withLabel schemaCacheLoads "SUCCESS" incCounter
     setGauge schemaCacheQueryTime resTime
   SchemaCacheErrorObs{} -> do