Limit keyword not release memory after delete oldest metrics.

Hi @jaqx0r, I set the `Limit` keyword so mtail will remove the oldest metrics, but I noticed the memory usage of mtail process will increase and not decrease after GC.

https://github.com/google/mtail/blob/e35f4c5d7b53b9947336850b4e11d10a78428dfe/internal/metrics/metric.go#L215C20-L215C20

`m.labelValuesMap` type is `map[string]*LabelValue`, the `delete` will not shrink the map size. see https://github.com/golang/go/issues/20135

If someone uses the `Limit` keyword, it means that the number of metrics he wants to limit may be very large. Do we need to consider the memory consumption of this part?

I changed the `m.labelValuesMap` type to `map[[256]byte]*LabelValue`, because golang will allocates data more than 128 bytes from heap and save pointers in the bucket of map, and after GC pointers at map will be replaced by nil pointer, so the memory pointed will be recollected. This solution may have some unconsidered issues, just for reference.