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 with bounded label cardinality; no changes to proving, auth, or request handling semantics beyond metric recording.

Overview
Adds HttpMetricsLayer to the shared prover_http_middleware! stack so the prover exposes HTTP-level Prometheus series: request totals (bounded method + status labels), end-to-end latency by method, and an in-flight gauge with an RAII guard so panics/cancellation don’t leak the gauge.

The layer sits below HealthLayer / MetricsLayer so /health and /metrics short-circuits never hit it and don’t skew latency. preregister_http_metrics() runs from install_exporter so series exist before the first real request (histogram is described but not pre-recorded, to avoid phantom zero observations).

Unit tests assert counter/histogram deltas and that in-flight returns to zero after sequential requests.

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[avi-starkware]

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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: log resolved config at startup (secrets redacted) #14684
• 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,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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