## Description
This PR refactors `scripts/metricsdocgen/main.go` to support merging static and generated metrics files for documentation generation.
The static `metrics` file remains necessary for metrics not defined in the coder codebase (`go_*`, `process_*`, `promhttp_*`, `coder_aibridged_*`), as well as **edge cases** the scanner cannot handle (e.g., such as metrics with runtime-determined labels or function-local variable references for fields, ...). Handling these edge cases in the scanner would make it significantly more complex, so we keep this hybrid approach to accommodate them. This means that in such cases, developers need to update the `metrics` file directly, meaning there is still a risk of out-of-date information in the documentation. However, this solution should already encompass most cases.
Static metrics take priority over generated metrics when both files contain the same metric name, allowing manual overrides without modifying the scanner. Some of these edge cases could be easily fixed by updating the codebase to use one of the supported patterns.
## Changes
* Update `scripts/metricsdocgen/main.go` to read from two separate metrics files:
* `metrics`: static, manually maintained metrics (e.g., `go_*`, `process_*`, `promhttp_*`, `coder_aibridged_*`)
* `generated_metrics`: auto-generated by the AST scanner
* Update `metrics` file to contain only static and edge-case metrics
* Skip metrics with empty HELP descriptions in the scanner
* Update `generated_metrics` to reflect skipped metrics
* Update `docs/admin/integrations/prometheus.md` with merged metrics
Related to: https://github.com/coder/coder/issues/13223
**Disclosure:** This PR was mainly developed with Claude Sonnet 4, with iterative review and refinement by @ssncferreira
Susana Ferreira