[postgres/ci] more coverage when asserting metric tags

+ refactor a bit the code with the new handy methods that were added to
AgentCheckTest
+ added a bunch of FIXMEs in the code where me mutate caps-variables
which are supposed to be constant. Not doing it now b/c it involves a
further refactoring of the whole check logic

checks.d/postgres.py

@@ -365,10 +365,12 @@ def _collect_stats(self, key, db, instance_tags, relations, custom_metrics):
         bgw_instance_metrics = self._get_bgw_metrics(key, db)
 
         if db_instance_metrics is not None:
+            # FIXME: constants shouldn't be modified
             self.DB_METRICS['metrics'] = db_instance_metrics
             metric_scope.append(self.DB_METRICS)
 
         if bgw_instance_metrics is not None:
+            # FIXME: constants shouldn't be modified
             self.BGW_METRICS['metrics'] = bgw_instance_metrics
             metric_scope.append(self.BGW_METRICS)
 
@@ -383,6 +385,7 @@ def _collect_stats(self, key, db, instance_tags, relations, custom_metrics):
 
         replication_metrics = self._get_replication_metrics(key, db)
         if replication_metrics is not None:
+            # FIXME: constants shouldn't be modified
             self.REPLICATION_METRICS['metrics'] = replication_metrics
             metric_scope.append(self.REPLICATION_METRICS)
 

tests/test_postgres.py

@@ -45,10 +45,7 @@ def test_checks(self):
             }
         ]
 
-        self.run_check(dict(instances=instances))
-        # Rate metrics, need 2 collection rounds
-        time.sleep(1)
-        self.run_check(dict(instances=instances))
+        self.run_check_twice(dict(instances=instances))
 
         # Useful to get server version
         # FIXME: Not great, should have a function like that available
@@ -219,4 +216,3 @@ def test_checks(self):
         )
 
         self.coverage_report()
-        return