Refactor slm collector (#932)

- Move metric Desc to vars to aid in unused linter checks
- Use new Collector interface

Signed-off-by: Joe Adams <[email protected]>

Author: sysadmind

CHANGELOG.md

@@ -1,3 +1,11 @@
+## master / unreleased
+
+BREAKING CHANGES:
+
+The flag `--es.slm` has been renamed to `--collector.slm`.
+
+* [CHANGE] Rename --es.slm to --collector.slm #XXX
+
 ## 1.8.0 / 2024-09-14
 
 * [FEATURE] Add tasks action collector. Enable using `--collector.tasks.actions`. #778