Goal: 

• PromQLRbac in the observabilityclient is sending requests to Prometheus to get metric names. This needs to be cached, so that it's not sent with each call to modify_query
• https://opendev.org/openstack/python-observabilityclient/src/branch/master/observabilityclient/rbac.py#L93

Acceptance Criteria:

• Change is proposed and passes CI