docs: Remove metrics automation per reviewer feedback

As requested by @charlieegan3:
- Remove auto-generation tooling (cmd/metrics-docs/main.go)
- Remove auto-generated metrics registry file
- Remove Makefile target for metrics generation

The reviewer indicated metrics don't change frequently enough
to warrant automation, and prefers avoiding duplicate lists.

Signed-off-by: Anivar A Aravind <ping@anivar.net>

Author: anivar

Makefile

@@ -579,6 +579,3 @@ depr-release-local:
 		$(RELEASE_BUILD_IMAGE) \
 		/_src/build/build-release.sh --output-dir=/$(RELEASE_DIR) --source-url=/_src
 
-.PHONY: generate-metrics-docs
-generate-metrics-docs:
-	@$(GO) run cmd/metrics-docs/main.go > docs/docs/metrics-registry.md