perf(tsdb): don't fetch labels in TSDBIndex.GetChunkRefs

[bboreham]

What this PR does / why we need it:

I noticed in a profile from production that index-gateways were spending a lot of time reading labels, in a context where labels are not typically needed.

This PR is made up of a refactoring to simplify the change, then adding the possibility to not fetch labels, and finally changing GetChunkRefs to only fetch labels when it needs to. It may need labels for series filtering, which is used by some downstream projects.

The benchmark results are not impressive, because BenchmarkTSDBIndex_GetChunkRefs only creates three series.
Still, a 30% drop in objects allocated shows that something was achieved.

I fixed the benchmark to return memory to the pool, otherwise that dominates.

goos: linux
goarch: amd64
pkg: github.com/grafana/loki/v3/pkg/storage/stores/shipper/indexshipper/tsdb
cpu: Intel(R) Core(TM) i7-14700K
                          │ before.txt  │             after.txt             │
                          │   sec/op    │   sec/op     vs base              │
TenantHeads/10-28           105.1µ ± 3%   106.4µ ± 2%       ~ (p=0.093 n=6)
TenantHeads/100-28          775.7µ ± 4%   717.7µ ± 5%  -7.49% (p=0.002 n=6)
TenantHeads/1000-28         7.897m ± 6%   7.650m ± 3%       ~ (p=0.132 n=6)
IndexClient_Stats-28        17.55µ ± 1%   17.67µ ± 1%       ~ (p=0.093 n=6)
TSDBIndex_GetChunkRefs-28   860.3µ ± 1%   836.5µ ± 2%  -2.77% (p=0.009 n=6)
TSDBIndex_Volume-28         280.0µ ± 1%   280.2µ ± 1%       ~ (p=0.937 n=6)
geomean                     373.7µ        366.4µ       -1.95%

                          │  before.txt  │             after.txt              │
                          │     B/op     │     B/op      vs base              │
TenantHeads/10-28           627.4Ki ± 0%   627.1Ki ± 0%       ~ (p=0.093 n=6)
TenantHeads/100-28          6.094Mi ± 0%   6.077Mi ± 0%  -0.28% (p=0.002 n=6)
TenantHeads/1000-28         63.20Mi ± 1%   63.33Mi ± 0%       ~ (p=0.818 n=6)
IndexClient_Stats-28        2.685Ki ± 2%   2.700Ki ± 2%       ~ (p=0.485 n=6)
TSDBIndex_GetChunkRefs-28   1.343Mi ± 0%   1.342Mi ± 0%  -0.07% (p=0.002 n=6)
TSDBIndex_Volume-28         67.22Ki ± 0%   67.22Ki ± 0%       ~ (p=0.061 n=6)
geomean                     630.6Ki        631.0Ki       +0.07%

                          │ before.txt  │              after.txt               │
                          │  allocs/op  │  allocs/op   vs base                 │
TenantHeads/10-28           2.135k ± 0%   2.125k ± 0%   -0.47% (p=0.002 n=6)
TenantHeads/100-28          21.34k ± 0%   21.23k ± 0%   -0.49% (p=0.002 n=6)
TenantHeads/1000-28         213.5k ± 0%   212.5k ± 0%   -0.47% (p=0.002 n=6)
IndexClient_Stats-28         55.00 ± 0%    55.00 ± 0%        ~ (p=1.000 n=6) ¹
TSDBIndex_GetChunkRefs-28    27.00 ± 0%    19.00 ± 0%  -29.63% (p=0.002 n=6)
TSDBIndex_Volume-28         3.060k ± 0%   3.060k ± 0%        ~ (p=1.000 n=6) ¹
geomean                     1.880k        1.769k        -5.91%
¹ all samples are equal

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• NA Documentation added
• NA Tests updated
• Title matches the required conventional commits format, see here
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• NA 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.