starknet_transaction_prover: document observability surface in README

[avi-starkware]

Adds a top-level Observability section to the README covering the
HTTP-only side endpoints (/health, /metrics), the metric names and
label sets, request-id propagation rules, the per-request log shape, the
startup banner, and the graceful-shutdown / panic-hook contract.

Updates the Environment variables table with LOG_FORMAT and
HEALTH_MAX_SATURATED_MS, the JSON config table with
health_max_saturated_ms, and the Logging subsection with the JSON
format option and URL redaction note.

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

[cursor]

PR Summary

Low Risk
README-only changes; no runtime, API, or security behavior is modified.

Overview
Documentation-only update to crates/starknet_transaction_prover/README.md so operators can run the prover with health checks, metrics, and structured logging without reading the code.

The API reference now points at GET /health and GET /metrics beside the JSON-RPC surface. Config tables add LOG_FORMAT, HEALTH_MAX_SATURATED_MS, and JSON health_max_saturated_ms. The Logging section covers JSON log output and URL credential redaction.

A new Observability section documents probe behavior (including saturation-driven 503 tied to -32005), Prometheus metric names and bounded labels, x-request-id handling, per-request log shape, startup banner redaction, and graceful shutdown / panic-hook behavior.

^{Reviewed by Cursor Bugbot for commit f6f9a32. Bugbot is set up for automated code reviews on this repo. Configure here.}

[reviewable-StarkWare]

This change is 

[avi-starkware]

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Learn more

• starknet_transaction_prover: document observability surface in README #14203 👈 (View in Graphite)
• 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
• 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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