telemetry: move geoprobe-agent metrics into geoprobe package

[nikw9944]

Fixes #3432

Summary

• Move geoprobe metrics from the shared internal/metrics package into a Metrics struct in internal/geoprobe, replacing package-level promauto globals with explicit dependency injection via NewMetrics()
• Add source and device_pubkey constant labels to all geoprobe metrics, enabling per-device filtering in dashboards
• Shorten metric names from doublezero_device_geoprobe_agent_* to doublezero_geoprobe_*
• Delete the now-unused geolocation_metrics.go from the shared metrics package
• Moving geoprobe metrics from package-level globals into a struct with source and device_pubkey constant labels lets multiple binaries emit the same metric names without collision, so dashboards can filter by source and device without needing separate metric prefixes per consumer.

Lines of Code

File	Lines	Status
geoprobe/metrics.go	190	New
geoprobe/metrics_test.go	144	New (test)
geoprobe-agent/main.go	~30	Modified
metrics/geolocation_metrics.go	139	Deleted
Net new (excl. tests)	~51

Testing Verification

• Unit tests verify all 12 collectors are registered, constant labels are applied correctly, and duplicate registration panics as expected
• All existing offsetCache tests in geoprobe-agent continue to pass unchanged
• go vet clean across the telemetry module

[ben-dz]

The new constant label is named device_pubkey, but the geoprobe-agent runs on geoprobe servers, not on DZDs/devices. The value passed is geoProbePubkey.String() — that's a geoprobe pubkey, not a device pubkey. This will cause confusion in PromQL queries and dashboards. Should probably be geoprobe_pubkey or just pubkey.

NewMetrics() is called unconditionally at

doublezero/controlplane/telemetry/cmd/geoprobe-agent/main.go

Line 340 in 9831f4c

// Set up prometheus metrics.

, before the if *metricsEnable check. All 12 collectors get registered to prometheus.DefaultRegisterer even if no HTTP server is started to serve them. Not a regression (promauto did the same), but might want to fix it.