metrics: runtime GC panel is not intuitive when GC < 1

### What problem does this feature solve?

The `Runtime GC rate and GOMEMLIMIT` panel is not very intuitive today.

The GC series can show values smaller than `1`, and when that happens it is hard to immediately understand what the panel is trying to tell the user:

- Is it showing GC count per minute?
- Is it showing an extrapolated rate?
- Is a value like `0.2` good, bad, or simply expected?

As a result, the panel does not communicate its intent clearly enough during triage.

### What is changed and how it works?

This issue is for improving the dashboard UX and metric presentation for the runtime GC panel so that:

- the meaning of the GC line is easier to understand at a glance
- values smaller than `1` do not feel ambiguous
- users can more quickly map the panel to a concrete operational question

### Related context

There is already a follow-up fix to tighten the selector used by the total GC query in the runtime dashboard. This issue is about the usability / readability problem of the panel itself.

### Suggested labels

- `type/enhancement`
- `component/metrics`