starknet_transaction_prover: HTTP request count + latency + in-flight metrics

[avi-starkware]

Adds HttpMetricsLayer placed between the monitoring endpoints and the
rest of the stack so probes/scrapes don't distort the request-latency
distribution. Records request count by bounded method/status labels,
end-to-end latency, and in-flight gauge. Also adds a shared test recorder
helper that installs the global Prometheus recorder once across the
test binary.

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

[cursor]

PR Summary

Low Risk
Observability-only middleware and test harness changes; no auth, proving, or request-handling logic is modified.

Overview
Adds HttpMetricsLayer to the HTTP and HTTPS middleware stacks below HealthLayer / MetricsLayer so health and /metrics scrapes are not counted in request latency or traffic metrics.

The layer emits prover_http_requests_total (bounded method + status class labels), prover_http_request_duration_seconds, and prover_http_inflight_requests with an RAII guard so in-flight counts drop on panic/cancel. Series are pre-registered at exporter install (without seeding the histogram with a zero observation).

Tests use a new test_recorder::shared_handle() (OnceLock) so the global Prometheus recorder is installed once across the crate’s unit tests.

^{Reviewed by Cursor Bugbot for commit 83d004c. 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 👈 (View in Graphite)
• starknet_transaction_prover: proving-job duration + outcome metrics #14168
• starknet_transaction_prover: Prometheus /metrics endpoint with build_info #14167
• starknet_transaction_prover: global panic hook + graceful SIGTERM shutdown #14166
• 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

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[reviewable-StarkWare]

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