`update_p4prom.sh` should disable `monitor_metrics.timer` when `p4metrics` is installed

[jackclucas]

When upgrading from the deprecated monitor_metrics.sh/systemd timer setup to
the p4metrics daemon, both end up running simultaneously and writing to the
same p4_monitor-<instance>-<serverid>.prom file.

This causes intermittent stale/missing metrics — most noticeably
p4_monitor_max_cmd_time disappearing and p4_process_count showing 0 —
because whichever process writes last wins.

The monitor_metrics.sh script header says it's deprecated in favour of
p4metrics, but the install/update scripts don't disable the old timer.

Fix: install_p4prom.sh and update_p4prom.sh should detect and
systemctl disable --now monitor_metrics.timer monitor_metrics.service
as part of the migration when p4metrics is being installed/enabled.