mgr/prometheus: expose daemon health metrics

mgr/prometheus: expose daemon health metrics

Until now daemon health metrics were stored without being used. One of
the most helpful metrics there is SLOW_OPS with respect to OSDs and MONs
which this commit tries to expose to bring fine grained metrics to find
troublesome OSDs instead of having a lone healthcheck of slow ops in the
whole cluster.

Signed-off-by: Pere Diaz Bou <pdiazbou@redhat.com>