feat(metrics): expose ingestion-lag gauge at /metrics (Closes #237)

[codenerde]

Summary

Completes #237 — the ingestion-lag gauge (sorotrail_ingestion_lag_ledgers) is now actually served at the /metrics endpoint and covered by table-driven tests.

The gauge itself (latest_rpc_ledger − last_ingested_ledger) is already tracked by the ingester via setIngestionLag, called after every poll pass that knows the chain head. What was missing:

1. Exposure at /metrics — the API server's /metrics handler (HTTPMetrics.Handler) only gathered its per-server histogram registry, so the globally-registered sorotrail_ingestion_lag_ledgers gauge was never exported. It now gathers prometheus.DefaultGatherer alongside the per-server registry, so a single scrape sees the whole picture.
2. Repair of a pre-existing compile break — Handler() in internal/metrics/metrics.go was missing its closing brace (leftover from an earlier bad merge), so the package did not compile. Fixed here.
3. Tests — table-driven coverage of the gauge value and the /metrics export path.

Tests

• internal/metrics/metrics_test.go — TestIngestionLagGauge_ExportedAtMetrics: asserts the exact chain-head-minus-last-ingested value is exported at /metrics (caught up, small lag, deep lag, nothing-ingested-yet).
• internal/ingester/ingester_test.go — TestSetIngestionLag: asserts setIngestionLag computes chainHead − lastIngested exactly, is a no-op when either side is unknown (≤ 0), and preserves negative lag when a replay runs ahead of the reported head.
• internal/api/server_test.go — TestMetrics_ExposesIngestionLagGauge: end-to-end assertion that the API /metrics endpoint serves the gauge.

All new tests pass locally.

Validation

• go build ./... ✅
• go vet ./... ✅
• golangci-lint run — no new issues from this change; the command currently reports pre-existing findings in unrelated in-flight work (cmd/sorotrail/main.go, internal/api/*, internal/ingester/ingester.go unused symbols / goimports).
• go test ./... — the packages touched by this change pass; the full suite has pre-existing failures in unrelated WIP code (two TestEventsIngestedTotal_* cases and pkg/docs openapi drift) that predate this PR and are out of scope for Add an ingestion-lag gauge #237.

Scope

4 files changed, +91/−18. Purely observability: no changes to endpoints, config, or schema.

Closes #237