[Feat]: Add notification on child node going stale

[mkesper]

Problem

Two nodes configured, one as child and one as parent. Configured alerts and notifications are working properly. No alert is triggered when the child cannot reach the parent, though.

Description

An alert should be triggered by default when a child agent vanishes as then this node will go completely unmonitored! Overriding to ignore ephemeral nodes should be added.

Importance

must have

Value proposition

1. Without this feature, relying on netdata agent for node monitoring is misleading (No data sent -> no alert)
2. Not having this feature would require adding further external checks, complicating the setup.

Proposed implementation

Check every minute (by default) whether child nodes are live. If not, go into alarm.

[netdata-community-bot]

This issue has been mentioned on the Netdata Community Forums. There might be relevant details there:

https://community.netdata.cloud/t/alert-when-non-ephemeral-nodes-go-stale/5108/5

[wmertens]

I believe this could be addressed by #17539