Add MESHCORE_TELEMETRY_POLL_24H_EXEMPT cooldown exemption for telemetry

[nicjansma]

I've recently started playing with MeshCore in addition to a few Meshtastic nodes I have deployed.

For Meshtastic, I run a solar node and use Potato Mesh to gather telemetry and alert on its health (e.g. using prometheus metrics).

I've now deployed a MeshCore node, and wanted to track its health (e.g. battery) as well. I know MeshCore has a different telemetry philosophy, i.e. poll vs. a push model. This is controlled by MESHCORE_TELEMETRY_POLL_SECONDS, but the current behavior is "each contact is additionally polled at most once per 24 h".

I'd like to get more granular health data, i.e. every 5 minutes like I get from my Meshtastic nodes. I don't want this from every connectable MeshCore node, just my specific telemetry devices.

To do this I've added a MESHCORE_TELEMETRY_POLL_24H_EXEMPT config variable that is an allow-list of nodes to poll more frequently. They will not be limited to one poll every 24h. The MESHCORE_TELEMETRY_POLL_SECONDS still applies, and only gathers a single telemetry request each poll interval, so nodes in MESHCORE_TELEMETRY_POLL_24H_EXEMPT will approximately only get metrics each MESHCORE_TELEMETRY_POLL_SECONDS * num_eligible_nodes seconds (the exempt nodes, plus any other contact whose 24 h cooldown has lapsed that rotation).

AI Disclosure

This PR was co-authored with Claude. I have independently reviewed, tested and tweaked the output.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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