mgr/dashboard: fix stale metrics in hardware dashboards

Switch all stat panels in the hardware and hardware compression
dashboards from lastNotNull to last reduceFunction. When a host is
unreachable or node-proxy stops, metric silence is ambiguous — it
could mean host down, agent crash, or scrape failure. lastNotNull
freezes the last known value (e.g. 4 NVMe drives), causing
inconsistency with the global Overview which reflects live count
reductions.

Use last so panels show N/A when no current data arrives.

Fixes: https://tracker.ceph.com/issues/78360

Signed-off-by: Afreen Misbah <afreen@ibm.com>