remove instance-level database tag from DBM metrics & events (#9469)

Remove the incorrect `db` tag inherited from the check instance tags from postgres DBM statement metrics & FQT events. This tag needs to be excluded because statement metrics & FQT events are collected from all databases on the host as of #9252. DBM statement samples are already doing the right thing here.

postgres/datadog_checks/postgres/statements.py

@@ -135,6 +135,10 @@ def _db_hostname_cached(self):
         return self._db_hostname
 
     def collect_per_statement_metrics(self, db, db_version, tags):
+        # exclude the default "db" tag from statement metrics & FQT events because this data is collected from
+        # all databases on the host. For metrics the "db" tag is added during ingestion based on which database
+        # each query came from.
+        tags = [t for t in tags if not t.startswith("db:")]
         try:
             rows = self._collect_metrics_rows(db)
             if not rows:

postgres/tests/test_pg_integration.py

@@ -288,7 +288,8 @@ def _should_catch_query(dbname):
     assert event['host'] == 'stubbed.hostname'
     assert event['timestamp'] > 0
     assert event['min_collection_interval'] == dbm_instance['min_collection_interval']
-    assert set(event['tags']) == {'foo:bar', 'server:{}'.format(HOST), 'port:{}'.format(PORT), 'db:datadog_test'}
+    expected_dbm_metrics_tags = {'foo:bar', 'server:{}'.format(HOST), 'port:{}'.format(PORT)}
+    assert set(event['tags']) == expected_dbm_metrics_tags
     obfuscated_param = '?' if POSTGRES_VERSION.split('.')[0] == "9" else '$1'
 
     dbm_samples = aggregator.get_event_platform_events("dbm-samples")
@@ -325,6 +326,10 @@ def _should_catch_query(dbname):
         assert fqt_event['postgres']['rolname'] == username
         assert fqt_event['timestamp'] > 0
         assert fqt_event['host'] == 'stubbed.hostname'
+        assert set(fqt_event['ddtags'].split(',')) == expected_dbm_metrics_tags | {
+            "db:" + fqt_event['postgres']['datname'],
+            "rolname:" + fqt_event['postgres']['rolname'],
+        }
 
     for conn in connections.values():
         conn.close()