feat: Optimize log parsing performance by using unsafe package

[cyriltovena]

This commit modifies the log parsing logic in the distributor package to improve performance. It introduces the use of the unsafe package to work with log strings as byte slices, reducing memory allocations and improving efficiency.

Benchstat

name                                          old time/op    new time/op    delta
_optParseExtractLogLevelFromLogLineLogfmt-10    1.25µs ± 1%    0.66µs ± 7%  -46.65%  (p=0.008 n=5+5)

name                                          old alloc/op   new alloc/op   delta
_optParseExtractLogLevelFromLogLineLogfmt-10    4.15kB ± 0%    0.02kB ± 0%  -99.61%  (p=0.008 n=5+5)

name                                          old allocs/op  new allocs/op  delta
_optParseExtractLogLevelFromLogLineLogfmt-10      5.00 ± 0%      1.00 ± 0%  -80.00%  (p=0.008 n=5+5)

name                                        old time/op    new time/op    delta
_optParseExtractLogLevelFromLogLineJson-10    1.20µs ± 0%    0.93µs ±20%  -22.34%  (p=0.008 n=5+5)

name                                        old alloc/op   new alloc/op   delta
_optParseExtractLogLevelFromLogLineJson-10      293B ± 0%       16B ± 0%     ~     (p=0.079 n=4+5)

name                                        old allocs/op  new allocs/op  delta
_optParseExtractLogLevelFromLogLineJson-10      6.00 ± 0%      1.00 ± 0%  -83.33%  (p=0.008 n=5+5)