Label RPC errors by network, and export the chain tip #188

Background

rpcErrorsTotal (src/metrics.ts:58-63) has only an outcome label, and recordRpcError is called from src/rpc.ts:167, 170 without a network. With two loops running you cannot tell which chain's provider is failing — which is the entire point of the metric.

Separately, last_indexed_ledger is exported (src/metrics.ts:71-76) but the chain tip is not, so Prometheus cannot compute indexer lag. /status computes it (src/api.ts:445), but that is a JSON endpoint, not a scrape target.

Acceptance criteria

• rpc_errors_total gains a network label, threaded from the withRetry callers
• A chain_tip_ledger{network} gauge is set on each successful tip read
• The README /metrics sample block is updated
• Tests assert the new labels and series appear in renderMetrics() — fails before

Drips Wave · Complexity: Easy · 100 points

How this repo is laid out

Wraith is a Soroban incoming-transfer indexer: it fills the gap Horizon leaves for SAC/SEP-41 token events by recipient address. Roughly 14k lines under src/, REST + GraphQL + WebSocket, Postgres via Prisma, deployed on Render.

• Unit tests are jest (src/**/__tests__, tests/*.test.ts), with a coverage gate.
• Integration tests are vitest (tests/integration/, vitest.integration.config.ts, Docker).
• Don't mix the two runners in one file.

Known state of main before you start

tsc --noEmit and the unit suite are green. The Integration tests CI job is red, and so is Load test — both predate this batch. Open PRs #177 and #178 address them. If the integration job is red when you open a PR, that is not your fault; say so in the PR and carry on.

Four modules are shipped, closed as done, and imported by nothing: src/indexer/checkpoint.ts, src/ingest/reorg.ts, src/middleware/opa.ts, and src/api/candles.ts + src/workers/ohlc-refresh.ts. So the live indexer today has no reorg handling and no atomic checkpointing, despite both features being closed. Two issues below start unpicking that — don't assume a closed issue means a wired feature.

Ground rules

• Every PR needs a test that fails before the change and passes after, unless the issue says otherwise.
• Don't widen a Prometheus label to something unbounded — addresses and contract ids must never become label values.
• Nothing may log or return a raw database error, a connection string, or a provider URL.
• Don't reformat a file you're changing. A diff that is 90% whitespace hides the 10% that matters.

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