Add Prometheus metrics exporter

[Otaiki1]

Context

Grafana dashboards require Prometheus metrics.

Goal

Add @otel/sdk or similar to expose metrics at /metrics.

Contributor guide

1. Directory: indexer/
2. Define counters for events_processed_total, errors_total.
3. Define gauges for indexer_lag_ledgers, memory_usage.
4. Configure scrape target in Prometheus config.
5. Add grafana dashboard template.

References

• Prometheus Node.js client docs.

[Kenlachy]

@Kenlachy has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I would like to work on this issue because adding a Prometheus metrics exporter improves observability, enabling real-time monitoring, alerting, and performance analysis. It allows teams to track system health, detect anomalies, and make data-driven decisions.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Kenlachy to this issue.