mgr/prometheus: introduce metric for collection time

mgr/prometheus: introduce metric for collection time

Introduces metric `prometheus_collect_duration_seconds` for the time it
takes the Prometheus manager module to collect all the metrics.

```
ceph_prometheus_collect_duration_seconds_sum{method="get_health"} 0.0002613067626953125
ceph_prometheus_collect_duration_seconds_sum{method="get_pool_stats"} 0.0018298625946044922
ceph_prometheus_collect_duration_seconds_sum{method="get_df"} 0.0005767345428466797
ceph_prometheus_collect_duration_seconds_sum{method="get_fs"} 0.0010402202606201172
ceph_prometheus_collect_duration_seconds_sum{method="get_quorum_status"} 0.0007524490356445312
ceph_prometheus_collect_duration_seconds_sum{method="get_mgr_status"} 0.0035364627838134766
ceph_prometheus_collect_duration_seconds_sum{method="get_pg_status"} 0.00021266937255859375
ceph_prometheus_collect_duration_seconds_sum{method="get_osd_stats"} 0.0018737316131591797
ceph_prometheus_collect_duration_seconds_sum{method="get_metadata_and_osd_status"} 0.0032796859741210938
ceph_prometheus_collect_duration_seconds_sum{method="get_num_objects"} 0.00011086463928222656
ceph_prometheus_collect_duration_seconds_sum{method="get_rbd_stats"} 0.00036144256591796875
ceph_prometheus_collect_duration_seconds_count{method="get_health"} 1.0
ceph_prometheus_collect_duration_seconds_count{method="get_pool_stats"} 1.0
ceph_prometheus_collect_duration_seconds_count{method="get_df"} 1.0
ceph_prometheus_collect_duration_seconds_count{method="get_fs"} 1.0
ceph_prometheus_collect_duration_seconds_count{method="get_quorum_status"} 1.0
ceph_prometheus_collect_duration_seconds_count{method="get_mgr_status"} 1.0
ceph_prometheus_collect_duration_seconds_count{method="get_pg_status"} 1.0
ceph_prometheus_collect_duration_seconds_count{method="get_osd_stats"} 1.0
ceph_prometheus_collect_duration_seconds_count{method="get_metadata_and_osd_status"} 1.0
ceph_prometheus_collect_duration_seconds_count{method="get_num_objects"} 1.0
ceph_prometheus_collect_duration_seconds_count{method="get_rbd_stats"} 1.0
```

Fixes: https://tracker.ceph.com/issues/46703
Signed-off-by: Patrick Seidensal <pseidensal@suse.com>