poll: fix swapped machine<->namespace labels

[hdhoang]

• read the CONTRIBUTING guidelines
• raised a GitHub issue or discussed it on the projects chat beforehand
• added unit tests
• added integration tests
• updated documentation if needed
• updated CHANGELOG.md

Hi, we noticed label mismatches in update_request_* metrics:

# correct
headscale_last_update_seconds{machine="router1",namespace="routers"} 1.644561297e+09
headscale_last_update_seconds{machine="router2",namespace="routers"} 1.644561296e+09

# swapped labels
headscale_update_request_from_node_total{machine="routers",namespace="router1",state="endpoint-update"} 47
headscale_update_request_from_node_total{machine="routers",namespace="router1",state="full-update"} 26
headscale_update_request_from_node_total{machine="routers",namespace="router1",state="scheduled-update"} 1096
headscale_update_request_from_node_total{machine="routers",namespace="router2",state="endpoint-update"} 4
headscale_update_request_from_node_total{machine="routers",namespace="router2",state="full-update"} 2
headscale_update_request_from_node_total{machine="routers",namespace="router2",state="scheduled-update"} 1058

Hope this helps!