## Description
This PR wires up the metrics scanner in the Makefile to automatically regenerate metrics documentation when source files change.
## Changes
* Add Makefile target `scripts/metricsdocgen/generated_metrics` to run the AST scanner to generate the metrics file
* Update `docs/admin/integrations/prometheus.md` Makefile target to depend on `scripts/metricsdocgen/generated_metrics`
* Add `scripts/metricsdocgen/README.md` documenting the metrics generation process
Closes: https://github.com/coder/coder/issues/13223