Include PodSpec.nodeName as a label in goldpinger_peers_response_time_s_* metrics

Hey guys!

Happy New Year!!! 🎉

Super cool project, we've recently deployed this in our clusters and it's proving to be super useful.

I was wondering if it would be possible to add a `nodeName` label to these (`goldpinger_peers_response_time_s_*`) metrics so it would be easier to identify the target nodes being probed.

the node name should be available in the `PodSpec.nodeName`

Start by locating the code that defines the goldpinger_peers_response_time_s_* metrics and the Kubernetes PodSpec handling. Confirm how the target pod's PodSpec.nodeName is available, then verify the metrics expose that value as nodeName without breaking existing labels or metric tests.

Written by the indexing model from the issue text.