mgr/prometheus: introduce fetch_perf_counters_metrics module option
By default, starting from ceph version `17.2.6`, prometheus module no more export perf counters as prometheus metrics. The new ceph-exporter daemon will be responsible for exporting these metrics instead. But in case if daemon deployment fails there should be an option to fetch these metrics.
Introduce a module option in prometheus module to enable/disable support for exporting ceph daemons perf counters as prometheus metrics, by default this option should be disabled.