grafana: add datasource input and templating for job/instance

[Illine]

Closes #902

This PR refactors the File.d Grafana dashboard to make it portable and configurable.

• Added Prometheus datasource input (DS_PROMETHEUS) and __inputs section
• Replaced hard-coded datasource UIDs with ${DS_PROMETHEUS}
• Introduced templating for job, instance, and pipeline
• Updated PromQL filters to use these variables (job="$job", instance=~"$instance")
• Added missing panel descriptions and replaced fixed windows with $__rate_interval (rates/histograms) and $__interval (over_time rollups)

[Illine]

Most of the diff is due to Grafana's JSON reformatting during export.
Functional changes are limited to inputs, templating variables, and query intervals.