Add configurable timeout for pending backend dials

[inerplat]

Summary

This PR adds an opt-in --backend-dial-timeout option to the proxy server.

When configured, the proxy server bounds how long a frontend dial can remain pending while sending DIAL_REQ to the selected backend or waiting for DIAL_RSP. On timeout, the pending dial is removed and the frontend receives a bounded error.

The default value is 0, so existing behavior is preserved unless operators explicitly enable the timeout.

Motivation

In HTTPConnect deployments used by kube-apiserver egress-selector setups, we observed a failure mode where backend connections remained registered while new HTTP CONNECT dials accumulated in pending_backend_dials. This can surface as admission webhook failures when the server-to-agent path stops making forward progress but the backend is still considered available.

The new option gives operators a guardrail against unbounded pending dial accumulation without changing default behavior.

Changes

• Add --backend-dial-timeout to proxy-server options.
• Keep the default disabled with 0.
• Apply the timeout while sending frontend DIAL_REQ packets to the selected backend.
• Apply the timeout while waiting for a backend DIAL_RSP before the HTTPConnect tunnel is established.
• Remove pending dial entries on timeout.
• Add a backend_dial_timeout dial failure metric reason.
• Add a reduced HTTPConnect regression test for blocked backend dial send.

Compatibility

The default behavior is unchanged because --backend-dial-timeout defaults to 0.

Operators can opt in with a value such as:

--backend-dial-timeout=30s

Testing

Ran:

go test ./cmd/server/app/options ./pkg/server -count=1

Added test:

TestHTTPConnectTunnelBlockedBackendDialSendTimesOutAndCleansPendingDial

The test starts the HTTPConnect tunnel handler, blocks backend Send(DIAL_REQ), verifies the dial enters pending state, and then verifies the configured timeout returns HTTP 504 and cleans up the pending dial.

Related Issue

#855

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[cheftako]

/ok-to-test

[inerplat]

@cheftako
The golangci-lint failure has been fixed. I also found and fixed an additional issue where a timed-out backend dial send could leave the backend connection wedged.

Could you please retrigger the CI for the latest commit?

Thanks.