fix(backend): stop leaking connection-metrics observers per connect

[garmr-ulfr]

Problem

StartConnectionMetrics was called on every vpn.Connected, allocating a fresh buffered event channel and OTel instruments each connect. The observers were never reclaimed on disconnect, so they accumulated on every connect/disconnect cycle.

Fix

• Build the connection-metrics observer once and reuse it across connects: attach it to the tunnel on Connected, detach (without destroying) on any other status.
• Fully release it — cancelling its recording goroutine — only when telemetry shuts down, via the new teardownConnMetrics.
• Also fixes stopTelemetry leaking the observer when telemetry was already disabled: the old updateConnMetrics(vpn.Disconnected) no-op'd once TelemetryKey was off.

[Copilot]

Pull request overview

This PR fixes a backend telemetry leak by ensuring connection-metrics observers are not re-created on every VPN reconnect and are properly released when telemetry shuts down.

Changes:

• Reuses a single connection-metrics observer across connect/disconnect cycles by attaching on vpn.Connected and detaching on other statuses.
• Adds teardownConnMetrics() to fully detach and cancel the metrics goroutine during telemetry shutdown, fixing a leak when telemetry is disabled.
• Refactors observer initialization into ensureConnMetricsObserverLocked() to centralize “create once” behavior.

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📥 Commits

Reviewing files that changed from the base of the PR and between 19f3684 and 553943f.

📒 Files selected for processing (1)

• backend/radiance.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-telemetry-conn-metrics-leak

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