starknet_transaction_prover: proving-job duration + outcome metrics

[avi-starkware]

Adds Prometheus counters / histograms recorded by VirtualSnosProver for
each proving job: total count by outcome (success, validation_error,
internal_error, l1_provider_error) and end-to-end duration.

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

[cursor]

PR Summary

Low Risk
Observability-only changes with a small refactor wrapper; no auth, data, or proving semantics changes.

Overview
Adds Prometheus instrumentation around prove_transaction and the proving pipeline so operators can track latency and failure mix without unbounded label cardinality.

Each request records a histogram for end-to-end wall time (prover_prove_transaction_duration_seconds) on success and failure, plus a counter with a fixed outcome label (prover_prove_transaction_outcome_total). Failures are bucketed via new VirtualSnosProverError::metric_outcome() into categories like validation, blocked, runner, output parse, and proving (when stwo_proving is enabled).

Successful runs that reach run_and_prove also emit sub-step histograms for virtual OS execution and Stwo proving. Metric names and outcome strings live in server/metrics (names / outcomes modules). Proving logic is unchanged aside from splitting prove_transaction into an inner method so metrics always run in a single wrapper; completion logs now include outcome.

^{Reviewed by Cursor Bugbot for commit a0e7299. 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: 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 👈 (View in Graphite)
• 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

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

[reviewable-StarkWare]

This change is