Send database instance metadata prior to check queries (#17590)

* Send database instance metadata prior to check queries

postgres/changelog.d/17590.fixed

@@ -0,0 +1 @@
+Send database instance metadata prior to check queries. This prevents a scenario where exceptions thrown during check execution can cause the database_instance resource to not be emitted. When the resource is not emitted, this can cause flapping in host tags. For example, a customer might see dbms:N/A for a period of time until a new database_instance resource is created. Moving this means it will always be sent, even if an unexepected exception is thrown during check execution. 

postgres/datadog_checks/postgres/postgres.py

@@ -1012,7 +1012,7 @@ def debug_stats_kwargs(self, tags=None):
     def check(self, _):
         tags = copy.copy(self.tags)
         self.tags_without_db = [t for t in copy.copy(self.tags) if not t.startswith("db:")]
-        # Collect metrics
+        tags_to_add = []
         try:
             # Check version
             self._connect()
@@ -1021,17 +1021,18 @@ def check(self, _):
 
             # Add raw version as a tag
             tags.append(f'postgresql_version:{self.raw_version}')
-            self.tags_without_db.append(f'postgresql_version:{self.raw_version}')
+            tags_to_add.append(f'postgresql_version:{self.raw_version}')
 
             # Add system identifier as a tag
             self.load_system_identifier()
             tags.append(f'system_identifier:{self.system_identifier}')
-            self.tags_without_db.append(f'system_identifier:{self.system_identifier}')
-
+            tags_to_add.append(f'system_identifier:{self.system_identifier}')
             if self._config.tag_replication_role:
                 replication_role_tag = "replication_role:{}".format(self._get_replication_role())
                 tags.append(replication_role_tag)
-                self.tags_without_db.append(replication_role_tag)
+                tags_to_add.append(replication_role_tag)
+            self._update_tag_sets(tags_to_add)
+            self._send_database_instance_metadata()
 
             self.log.debug("Running check against version %s: is_aurora: %s", str(self.version), str(self.is_aurora))
             self._emit_running_metric()
@@ -1044,7 +1045,6 @@ def check(self, _):
             if self._config.collect_wal_metrics:
                 # collect wal metrics for pg < 10, disabled by enabled
                 self._collect_wal_metrics()
-            self._send_database_instance_metadata()
         except Exception as e:
             self.log.exception("Unable to collect postgres metrics.")
             self._clean_state()
@@ -1069,3 +1069,7 @@ def check(self, _):
         finally:
             # Add the warnings saved during the execution of the check
             self._report_warnings()
+
+    def _update_tag_sets(self, tags):
+        self._non_internal_tags = list(set(self._non_internal_tags) | set(tags))
+        self.tags_without_db = list(set(self.tags_without_db) | set(tags))