[Feature Request] Add label for source_node_id in zerotier promethues metrics

Hi,
Currently, the Prometheus metrics exported by Zerotier don't include the source zerotier node which generated them. The documentation says to manually add this label in the scraping job config. This will become a lot of manual works if we want to monitor a lot of zerotier nodes. Also in case of running the zerotier nodes in Kubernetes as pods we will not be able to put this information on ServiceMonitor or PodMonitor due to dynamic nature of the pods.

I suggest enriching the metrics emitted by Zerotier to also include the source node id in the labels of the metrics which are specific to that node. As the node already has the information about its own id it can be a simple operation. For example, the updated metrics will then look like this:

zt_peer_path_count{source_id="wxy123456z",node_id="abc12345de",status="alive"} 0

Thank you.

Start by locating the C++ code that exports Prometheus metrics, especially the producer of zt_peer_path_count, and review how node-specific labels are currently assembled. Confirm the expected label name and scope from the issue example, then add coverage if an existing metrics test area is found; done means node-specific metrics include the source node ID without requiring scrape-time configuration.