Wrong prometheus metric labeling for limithandler

Prometheus metric labeling for ratelimit middleware is wrong, which is annoying for making related monitoring charts.

see

https://gitlab.com/gitlab-org/gitaly/-/blob/4ddad983e6a1ad0fdef147d6064ccf1a24dfac21/internal/middleware/limithandler/metrics.go#L16-34

and

https://gitlab.com/gitlab-org/gitaly/-/blob/4ddad983e6a1ad0fdef147d6064ccf1a24dfac21/internal/middleware/limithandler/metrics.go#L103-104

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