Add connection_duration_seconds histogram for post-dial connection lifespan

[HarnageaGabriel]

Summary

• Adds a connection_duration_seconds Prometheus histogram (buckets 1s–24h) tracking how long a proxied end-to-end connection stays established (post-dial) before it closes.
• Records the establish timestamp on ProxyClientConnection and observes the duration on both removal paths (removeEstablished, removeEstablishedForBackendConn), mirroring the existing dial_failure_count/reason instrumentation style.

Addresses one item of the checklist in #410 ("Connection lifespan (histogram)").

Test plan

• go build ./...
• go test ./pkg/server/... (68 tests pass, includes new TestConnectionDurationMetric)

🤖 Generated with Claude Code

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[kubernetes-prow]

Hi @HarnageaGabriel. Thanks for your PR.

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[cheftako]

/ok-to-test

[cheftako]

/lgtm
/approve

[kubernetes-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cheftako, HarnageaGabriel

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