mgr/prometheus: log time it takes to collect metrics

mgr/prometheus: log time it takes to collect metrics

When debug is enabled, the time it takes to gather the various types of
metrics is logged by type.

Fixes: https://tracker.ceph.com/issues/46444
Signed-off-by: Patrick Seidensal <pseidensal@suse.com>
(cherry picked from commit a5f86cb85fcbf8a9003bf511b57a6d6b4a0e1f41)

 Conflicts:
	src/pybind/mgr/mgr_util.py
  - Some more imports in master compared to octopus;
    had to add "import time" (line 6)  manually, otherwise make check
    would fail with a mypy error ('time is not defined')