mgr/prometheus: prune stale health checks, compress output

mgr/prometheus: prune stale health checks, compress output

This patch introduces several improvements to the Prometheus module:

 - Introduces `HealthHistory._prune()` to drop stale and inactive health checks.
  Limits the in-memory healthcheck dict to a configurable max_entries (default 1000).
  TTL for stale entries is configurable via `healthcheck_history_stale_ttl` (default 3600s).

 - Refactors HealthHistory.check() to use a unified iteration over known and current checks,
  improving concurrency and minimizing redundant updates.

 - Use cherrypy.tools.gzip instead of manual gzip.compress() for cleaner
  HTTP compression with proper header handling and client negotiation.

 - Introduces new module options:
    - `healthcheck_history_max_entries`

 - Add proper error handling for CherryPy engine startup failures
 - Remove os._exit monkey patch in favor of proper exception handling
 - Remove manual Content-Type header setting (CherryPy handles automatically)

Fixes: https://tracker.ceph.com/issues/68989
Signed-off-by: Nitzan Mordechai <nmordech@redhat.com>