docs: no Grafana dashboard setup guide for Prometheus + Grafana users

[Vaishnav88sk]

Describe your use-case which is not covered by existing documentation.

The existing docs/DASHBOARDS.md only covers Kibana (Elastic) dashboards. Users who run Prometheus + Grafana - a very common open-source stack - have no documentation showing how to:

• Configure the OTel Collector to export metrics to Prometheus
• Import a working Grafana dashboard for Jenkins health metrics
• Set up the customObservabilityBackend in the plugin to link builds to Grafana

The docs/monitoring-metrics.md file lists all available metrics but gives no guidance on how to visualise them in Grafana. Users are left to figure out the collector config, Prometheus scrape config, and dashboard panels entirely on their own.

Reference any relevant documentation, other materials or issues/pull requests that can be used for inspiration.

• docs/DASHBOARDS.md - only documents Kibana import, no Grafana content
• docs/monitoring-metrics.md - lists metrics but no Grafana visualisation guidance
• docs/setup-and-configuration.md - mentions customObservabilityBackend JCasC config but does not show a Grafana example end-to-end

Are you interested in contributing to the documentation?

Yes. I (@Vaishnav88sk) will submit a PR adding docs/grafana-dashboard-setup.md with:

• Full OTel Collector config for Prometheus export
• Prometheus scrape config
• A working Grafana dashboard JSON (provisioning-ready)
• Plugin JCasC config to link build pages back to Grafana
• Step-by-step import instructions