Some benchmarks for HEAD and allocate the correct slice size in LabelValues , we already know what it'll be
This is ~15% time improvement, and ~25% allocation improvement:
```
benchmark old ns/op new ns/op delta
BenchmarkHeadPostingForMatchers-4 74452 63514 -14.69%
benchmark old allocs new allocs delta
BenchmarkHeadPostingForMatchers-4 20 13 -35.00%
benchmark old bytes new bytes delta
BenchmarkHeadPostingForMatchers-4 5425 3137 -42.18%
```
Signed-off-by: Thomas Jackson <jacksontj.89@gmail.com>
Thomas Jackson