Add a Grafana/Prometheus dashboard template for Woodpecker #25
Description
In issue #16, we added detailed Prometheus metrics for all components of Woodpecker.
To enhance observability and make it easier to monitor the system, we now need a reference Grafana dashboard template. This template should cover key metrics such as WAL write throughput, read latency, segment state tracking, etc., and serve as a starting point for users or operators.
Goals:
- Provide an importable Grafana dashboard JSON template;
- Include visualizations for core Woodpecker metrics;
- Upload the template file to the user documentation.