Merge pull request #16406 from dmick/wip-mgr-counters

mgr: perf schema fns/change notification and Prometheus plugin

Reviewed-by: Sage Weil <sage@redhat.com>

diff --cc doc/release-notes.rst
Simple merge

diff --cc src/mgr/DaemonServer.cc
Simple merge

diff --cc src/mgr/PyModules.cc
Simple merge

diff --cc src/mgr/PyModules.h
Simple merge