added missing end_op

Author: daniel-sanche

google/cloud/bigtable/data/_async/_mutate_rows.py

@@ -144,9 +144,11 @@ async def start(self):
             all_errors: list[Exception] = []
             for idx, exc_list in self.errors.items():
                 if len(exc_list) == 0:
-                    raise core_exceptions.ClientError(
+                    exc = core_exceptions.ClientError(
                         f"Mutation {idx} failed with no associated errors"
                     )
+                    self._operation_metrics.end_with_status(exc)
+                    raise exc
                 elif len(exc_list) == 1:
                     cause_exc = exc_list[0]
                 else: