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 instrumentation around existing proving paths; no changes to validation, blocking check, or proof generation semantics.

Overview
Adds Prometheus observability around prove_transaction in VirtualSnosProver: end-to-end latency (prover_prove_transaction_duration_seconds), per-outcome counters (prover_prove_transaction_outcome_total with a fixed outcome label set in metrics::outcomes), and sub-step histograms for virtual OS run and Stwo proving.

Proving logic is unchanged functionally; the public prove_transaction now delegates to prove_transaction_inner and always records metrics and logs outcome on completion. VirtualSnosProverError::metric_outcome() maps error variants into those bounded labels so label cardinality stays stable (documented as requiring dashboard updates when variants change).

Note: The PR description’s outcome names (validation_error, internal_error, etc.) do not match the implemented labels (failure_validation, failure_runner, …).

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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 👈 (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,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

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[reviewable-StarkWare]

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