mgr/prometheus: Use RLock to fix deadlock in HealthHistory

mgr/prometheus: Use RLock to fix deadlock in HealthHistory

The HealthHistory.check() method acquires the lock and then calls
HealthHistory.save(), which also tries to acquire the same lock.
With a regular Lock(), the same thread blocks trying to re-acquire it (deadlock).
Switch to RLock to allow nested acquisition by the same thread.
PR #65245 added the locks.

Fixes: https://tracker.ceph.com/issues/74148
Signed-off-by: Nitzan Mordechai <nmordech@ibm.com>