refactor(stringlabels): Support stringlabels in chunkenc

[jeschkies]

What this PR does / why we need it:
This is a next step of support Prometheus stringlabels implementation in Loki. It adds support in the chunkenc package.

Special notes for your reviewer:
The tests should compile with build tag stringlabels.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[jeschkies]

TODO:

• Check BackwardIterator speed with benchmark

[jeschkies]

For some reason this shows up in allocations. Even though there should be none.

[jeschkies]

› benchstat before.log after.log
goos: linux
goarch: amd64
pkg: github.com/grafana/loki/v3/pkg/chunkenc
cpu: AMD Ryzen 7 3700X 8-Core Processor             
                           │ before.log  │             after.log              │
                           │   sec/op    │   sec/op     vs base               │
BackwardIterator/66_kB-16    17.97m ± 6%   18.23m ± 3%  +1.43% (p=0.007 n=10)
BackwardIterator/262_kB-16   15.83m ± 0%   15.84m ± 0%       ~ (p=0.796 n=10)
BackwardIterator/524_kB-16   12.96m ± 0%   12.75m ± 4%  -1.67% (p=0.003 n=10)
geomean                      15.45m        15.44m       -0.06%

                           │  before.log  │              after.log              │
                           │     B/op     │     B/op      vs base               │
BackwardIterator/66_kB-16    1.197Mi ± 1%   1.200Mi ± 0%       ~ (p=0.247 n=10)
BackwardIterator/262_kB-16   361.6Ki ± 2%   358.2Ki ± 1%       ~ (p=0.315 n=10)
BackwardIterator/524_kB-16   140.3Ki ± 2%   139.5Ki ± 1%       ~ (p=0.247 n=10)
geomean                      396.2Ki        394.5Ki       -0.43%

                           │ before.log  │             after.log              │
                           │  allocs/op  │  allocs/op   vs base               │
BackwardIterator/66_kB-16    2.764k ± 0%   2.764k ± 0%   0.00% (p=0.010 n=10)
BackwardIterator/262_kB-16    616.0 ± 0%    618.0 ± 0%  +0.32% (p=0.000 n=10)
BackwardIterator/524_kB-16    255.0 ± 0%    255.0 ± 0%       ~ (p=0.211 n=10)
geomean                       757.2         758.0       +0.11%

resolved.