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 documentation; no service behavior or deployment logic changes.

Overview
Expands starknet_transaction_prover README so operators can run the service with clear observability expectations—no runtime code changes.

The API reference now calls out GET /health and GET /metrics beside the JSON-RPC spec. Configuration docs add LOG_FORMAT / --log-format, HEALTH_MAX_SATURATED_MS / --health-max-saturated-ms, and health_max_saturated_ms in the JSON config table. The Logging subsection covers JSON log shape and URL redaction for credentials in rpc_node_url and blocking_check_url.

A new Observability section documents health probe semantics (503 when saturated for the configured window, tied to -32005), the Prometheus metric names and bounded labels, x-request-id handling (including the deliberate two-id model for OHTTP), the http_request log line, startup banner fields, and graceful shutdown / panic-hook behavior.

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[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,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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