Allocations are reduced by re-using the byte buffer when converting
labels to string keys.
```
name old allocs/op new allocs/op delta
GetStaleSeries-10 703 ± 0% 203 ± 0% ~ (p=1.000 n=1+1)
```
Signed-off-by: hagen1778 <roman@victoriametrics.com>
(cherry picked from commit b0c1f3d819)
