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

test/io/test_big_schema.py

@@ -34,7 +34,9 @@ def test_schema_cache_load_max_duration(defaultenv):
         assert match, f"unexpected log format: {schema_cache_lines[-1]}"
         duration_ms = float(match.group(1))
 
-        assert duration_ms < max_duration
+        # check that loading takes long enough
+        # to make sure we measure the time correctly
+        assert 100 < duration_ms < max_duration
 
 
 # TODO: This test fails now because of https://github.com/PostgREST/postgrest/pull/2122