Missing Metrics for last machine contact

[wrenix]

Bug description

after:
#1492

several metrics does not exists anymore.

i miss the metrics headscale_last_update_seconds which was nice for alerting if a machine was not connect correct with tailscale.

Environment

• OS:
• Headscale version: main
• Tailscale version: X

• Headscale is behind a (reverse) proxy
• Headscale runs in a container

To Reproduce

Logs and attachments

[kradalby]

Hi, this metric does not exist anymore due to a change in how we distribute updates. It will not return, but I am redoing the metrics that had been left and you can preview them in #1895.

The closest/most relevant for a similar purpose might be:

# HELP headscale_notifier_open_channels_total total count open channels in notifier
# TYPE headscale_notifier_open_channels_total gauge
headscale_notifier_open_channels_total 149

# HELP headscale_mapresponse_current_sessions_total total count open map response sessions
# TYPE headscale_mapresponse_current_sessions_total gauge
headscale_mapresponse_current_sessions_total 150