mgr/prometheus: Make standby discoverable

mgr/prometheus: Make standby discoverable

Enable config settings to modify standby's behaviour on the index page
This makes the standby discoverable by reverse proxy or loadbalancer
setups. Testing for the empty response of the '/metrics' endpoint would
trigger metric collection on the active manager instance.

The newly added configuration options settings standby_behaviour and
standby_error_status_code are documented and flagged as runtime, as
modifying both settings has an immediate effect (no restart required).

Co-authored-by: Ernesto Puerta <37327689+epuertat@users.noreply.github.com>
Signed-off-by: Roland Sommer <rol@ndsommer.de>
Fixes: https://tracker.ceph.com/issues/53229
(cherry picked from commit c1570f870e28d8fd4cd072832f1a2e19bac663eb)