chore(metastore): add column reader for pointers section

[ivkalita]

What this PR does / why we need it:

Adding columnar reader for (log)pointers section.

New reader is slower than existing RowReader due to double-translation of the data (columns->rows->rows vs columns->rows->columns->rows). The idea though is to get rid of the double-translation in the future.

Read section benchmarks

goos: darwin
goarch: arm64
pkg: github.com/grafana/loki/v3/pkg/dataobj/metastore
cpu: Apple M4
                                     │ result.old.txt │           result.new.txt            │
                                     │     sec/op     │    sec/op     vs base               │
ReadSections/single_index_file-10        157.2µ ±  5%   161.5µ ± 10%       ~ (p=0.105 n=10)
ReadSections/multiple_index_files-10     3.199m ± 11%   3.333m ± 10%  +4.19% (p=0.023 n=10)
geomean                                  709.1µ         733.8µ        +3.47%

                                     │ result.old.txt │           result.new.txt            │
                                     │      B/op      │     B/op      vs base               │
ReadSections/single_index_file-10        542.9Ki ± 0%   535.8Ki ± 0%  -1.31% (p=0.000 n=10)
ReadSections/multiple_index_files-10     28.74Mi ± 0%   27.75Mi ± 0%  -3.47% (p=0.000 n=10)
geomean                                  3.904Mi        3.810Mi       -2.39%

                                     │ result.old.txt │           result.new.txt           │
                                     │   allocs/op    │  allocs/op   vs base               │
ReadSections/single_index_file-10         2.369k ± 0%   2.498k ± 0%  +5.45% (p=0.000 n=10)
ReadSections/multiple_index_files-10      112.8k ± 0%   111.9k ± 0%  -0.87% (p=0.000 n=10)
geomean                                   16.35k        16.72k       +2.24%

RowReader vs Reader comparison benchmarks

                                                │ result.old.txt │            result.new.txt            │
                                                │     sec/op     │    sec/op     vs base                │
Readers/1k_pointers,_10_stream_IDs/Reader-10        265.8µ ±  5%   278.3µ ± 15%   +4.70% (p=0.001 n=10)
Readers/1k_pointers,_200_stream_IDs/Reader-10       686.2µ ± 12%   965.1µ ±  2%  +40.65% (p=0.000 n=10)
Readers/10k_pointers,_200_stream_IDs/Reader-10      695.1µ ±  2%   977.2µ ±  1%  +40.57% (p=0.000 n=10)
Readers/100k_pointers,_200_stream_IDs/Reader-10     714.4µ ± 16%   982.6µ ±  1%  +37.55% (p=0.000 n=10)
geomean                                             548.6µ         712.6µ        +29.90%

                                                │ result.old.txt │            result.new.txt             │
                                                │      B/op      │     B/op       vs base                │
Readers/1k_pointers,_10_stream_IDs/Reader-10        623.4Ki ± 0%    496.2Ki ± 0%  -20.41% (p=0.000 n=10)
Readers/1k_pointers,_200_stream_IDs/Reader-10       876.2Ki ± 0%   1443.3Ki ± 0%  +64.72% (p=0.000 n=10)
Readers/10k_pointers,_200_stream_IDs/Reader-10      914.1Ki ± 0%   1475.1Ki ± 0%  +61.37% (p=0.000 n=10)
Readers/100k_pointers,_200_stream_IDs/Reader-10     914.5Ki ± 0%   1475.4Ki ± 0%  +61.34% (p=0.000 n=10)
geomean                                             822.0Ki         1.091Mi       +35.92%

                                                │ result.old.txt │           result.new.txt            │
                                                │   allocs/op    │  allocs/op   vs base                │
Readers/1k_pointers,_10_stream_IDs/Reader-10         8.557k ± 0%   8.738k ± 0%   +2.11% (p=0.000 n=10)
Readers/1k_pointers,_200_stream_IDs/Reader-10        12.58k ± 0%   20.07k ± 0%  +59.54% (p=0.000 n=10)
Readers/10k_pointers,_200_stream_IDs/Reader-10       12.59k ± 0%   20.08k ± 0%  +59.46% (p=0.000 n=10)
Readers/100k_pointers,_200_stream_IDs/Reader-10      12.59k ± 0%   20.08k ± 0%  +59.46% (p=0.000 n=10)
geomean                                              11.43k        16.31k       +42.66%

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

[benclive]

Nice, LGTM, thanks!
I left one reply below about re-using the predicate, but I don't think it's a big deal so I'm happy for you to decide. Approving!