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 HTTP middleware and a counter on an existing rejection path; no changes to proving logic or auth.

Overview
Adds Prometheus observability to the transaction prover HTTP stack: a global recorder is installed at startup (before bind) and a MetricsLayer serves GET /metrics in Prometheus text format, mirroring how HealthLayer bypasses JSON-RPC.

Startup registers prover_build_info (version + git SHA labels) and pre-registers prover_concurrency_rejected_total. When the proving semaphore is full, prove_transaction increments that counter before returning the existing busy error.

The layer is wired through start_server and the HTTPS TLS path alongside health checks, with bounded label cardinality documented in the new metrics module. Dependencies metrics and metrics-exporter-prometheus are added; a unit test asserts scrape output includes build info.

^{Reviewed by Cursor Bugbot for commit b9b156a. Bugbot is set up for automated code reviews on this repo. Configure here.}

[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: redact URL credentials in logs and add startup banner #14164
• starknet_transaction_prover: add /health endpoint via HealthLayer #14163
• starknet_transaction_prover: add --log-format text|json CLI flag #14162
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[reviewable-StarkWare]

This change is 

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, have a team admin enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 9c4a5a0. Configure here.}