starknet_transaction_prover: bump prover_panics_total on panic hook fire

[avi-starkware]

Bumps the new prover_panics_total counter from the panic hook before
constructing the backtrace, so dashboards can alert on panic rate without
log search. The pre-registered zero observation in install_exporter
keeps the series visible at scrape time even before the first panic.

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

[cursor]

PR Summary

Low Risk
Observability-only change to the existing panic hook; unwinding and serving behavior are unchanged.

Overview
Adds a prover_panics_total Prometheus counter so operators can alert on panic rate without log queries.

The global panic hook now increments the counter before structured logging and backtrace capture, so a failure during logging cannot drop the count. install_exporter pre-registers the series at zero so /metrics always includes it. README observability and panic sections document the metric and recommend alerting on a rising rate.

Tests gain a mutex around panic-hook cases and a new check that each hooked panic bumps the counter by one.

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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: log resolved config at startup (secrets redacted) #14684
• starknet_transaction_prover: origin-level breadcrumbs for validation errors #14172
• starknet_transaction_prover: /health returns 503 when service is saturated #14171
• starknet_transaction_prover: track saturation and queue depth on the request path #15027
• starknet_transaction_prover: bump prover_panics_total on panic hook fire #14170 👈 (View in Graphite)
• starknet_transaction_prover: HTTP request count + latency + in-flight metrics #14169
• starknet_transaction_prover: proving-job duration + outcome metrics #14168
• starknet_transaction_prover: Prometheus /metrics endpoint with build_info #14167
• starknet_transaction_prover: extract the TLS accept loop and test its drain #15049
• starknet_transaction_prover: graceful shutdown on SIGTERM/SIGINT #15026
• starknet_transaction_prover: global panic hook with structured log #14166
• starknet_transaction_prover: document existing logging, request ids and health probe in README #14203
• main-v0.14.4

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