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 around the prover path; no auth, fee, or proving logic behavior changes beyond refactoring into an inner method.

Overview
Adds Prometheus observability around each prove_transaction call in VirtualSnosProver: end-to-end latency (prover_prove_transaction_duration_seconds), per-request outcome counts with a fixed outcome label set (prover_prove_transaction_outcome_total), and sub-step histograms for virtual OS execution and Stwo proving.

Proving logic moves into prove_transaction_inner so the public method can always record metrics and log outcome on completion. VirtualSnosProverError::metric_outcome() maps error variants to the bounded labels in server::metrics::outcomes (validation, blocked, runner, output parse, proving).

^{Reviewed by Cursor Bugbot for commit fb01d37. 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 👈 (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

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[reviewable-StarkWare]

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