admin: Track reverse_proxy in-flight requests

[pauloappbr]

Assistance Disclosure

I consulted an AI assistant (Gemini) to help refactor the UsagePool logic into the sync.RWMutex map approach suggested by @mholt. I fully reviewed, authored the final structure, and verified the code's correctness.

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This PR supersedes #7281 and implements the feature requested in #7277. Huge thanks to @u5surf for laying the groundwork!

As discussed in the previous PR and suggested by @mholt, this replaces the UsagePool implementation with a lightweight, package-level map[string]uint protected by a sync.RWMutex.

Changes:

• Removed inflightHosts UsagePool logic.
• Implemented inFlightRequests map to track requests during the proxy loop securely.
• Updated admin.go to expose these draining hosts alongside the static hosts in /reverse_proxy/upstreams.
• Fixed the double-counting issue pointed out by @huang041.
• Zero memory leak: hosts are deleted from the map as soon as their in-flight count reaches 0.

[pauloappbr]

@huang041 Brilliant catches! Thank you for the detailed review.You are totally right about the global lock contention under high RPS. I've updated the implementation to use sync.Map with atomic.Int64, which makes the hot path lock-free.Spot on regarding the $O(n^2)$ loop in admin.go. I've introduced a simple map[string]struct{} to track known hosts during the first iteration, reducing the subsequent lookup to $O(1)$.The changes have been pushed. Let me know if everything looks good now!

[mholt]

I'll review this soon... thanks!

[francislavoie]

While reviewing this it made me think about dynamic upstreams a bit more, and I came up with this #7539, which should pair nicely with the improved in-flight count tracking.

[pauloappbr]

Amazing work, @francislavoie! Breaking out the dynamic hosts into their own registry makes total sense.

The background cleanup goroutine is a very elegant approach. It acts like a TTL cache, preventing memory leaks while ensuring the health state persists long enough for the passive health checks to actually trip and do their job.

Really glad my refactor could help kick off this improvement! I'll be following this implementation closely to learn from it.

[mholt]

Neato, thank you for the revisions!