PR Summary

Low Risk
Observability-only refactor with metrics recorded after the same proving logic; no auth, data, or API behavior changes.

Overview
Adds Prometheus instrumentation around prove_transaction so operators can track latency and failure mix without changing proving semantics.

Each request records a bucketed end-to-end histogram (prover_prove_transaction_duration_seconds) on success or failure, plus a counter (prover_prove_transaction_outcome_total) labeled with a fixed outcome set (success, failure_validation, failure_blocked, failure_runner, failure_output_parse, failure_proving). Failures are classified via new VirtualSnosProverError::metric_outcome() so label cardinality stays bounded.

The proving path is split into prove_transaction (metrics + logging) and prove_transaction_inner (unchanged flow). run_and_prove now emits separate histograms for virtual OS run time and Stwo prove time. Metric name and outcome constants live in server::metrics.

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• 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: unify HTTP middleware stack via macro #14286
• main-v0.14.3

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