starknet_transaction_prover: Prometheus /metrics endpoint with build_info

[avi-starkware]

Adds MetricsLayer that short-circuits GET /metrics with a Prometheus
text-format scrape, and an install_exporter helper that registers the
global recorder and emits prover_build_info with version + git_sha
labels. Wired alongside HealthLayer so the scrape path bypasses the
JSON-RPC parser.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

[cursor]

PR Summary

Low Risk
Observability-only changes: new scrape endpoint and a counter on existing capacity rejection; no change to proving or auth logic.

Overview
Adds Prometheus observability to the transaction prover: startup calls install_exporter to register the global recorder and emit prover_build_info (version + git_sha) before the server binds, so early scrapes still see build identity.

A new MetricsLayer handles GET /metrics in the HTTP stack (alongside health), returning Prometheus text without hitting the JSON-RPC parser on both plain HTTP and TLS paths. prove_transaction now increments prover_concurrency_rejected_total when the concurrency semaphore rejects a request (counter is pre-registered at 0 at startup).

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[avi-starkware]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• starknet_transaction_prover: document observability surface in README #14203
• starknet_transaction_prover: origin-level breadcrumbs for validation errors #14172
• starknet_transaction_prover: /health returns 503 when service is saturated #14171
• starknet_transaction_prover: bump prover_panics_total on panic hook fire #14170
• starknet_transaction_prover: HTTP request count + latency + in-flight metrics #14169
• starknet_transaction_prover: proving-job duration + outcome metrics #14168
• starknet_transaction_prover: Prometheus /metrics endpoint with build_info #14167 👈 (View in Graphite)
• starknet_transaction_prover: global panic hook + graceful SIGTERM shutdown #14166
• starknet_transaction_prover,tower_ohttp: OHTTP-unlinkable request-id for decapsulated content #14222
• starknet_transaction_prover: per-request structured log with request-id #14165
• starknet_transaction_prover: unify HTTP middleware stack via macro #14286
• main-v0.14.3

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[reviewable-StarkWare]

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[cursor]

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