chore: Improve containsLower performance using quick rejection

[cyriltovena]

This pull request includes improvements to the containsLower function in pkg/logql/log/filter.go for better performance and adds benchmark tests to ensure the changes are effective. The key changes include optimizing the case-insensitive substring search and introducing comprehensive benchmark tests.

Optimizations to containsLower function:

• pkg/logql/log/filter.go: Optimized the containsLower function by implementing a more efficient search algorithm that first locates the potential starting byte of the substring and then verifies the rest of the substring with both fast ASCII and slower Unicode comparisons.

Addition of benchmark tests:

• pkg/logql/log/filter_test.go: Added a new benchmark function BenchmarkContainsLower with various test cases to evaluate the performance of the containsLower function under different scenarios, including short and long lines, matches and non-matches, and lines with Unicode characters.

Result:

benchstat before.txt before2.txt
name                                             old time/op    new time/op    delta
ContainsLower/short_line_no_match-10               43.2ns ± 1%    20.9ns ± 0%  -51.53%  (p=0.008 n=5+5)
ContainsLower/short_line_with_match-10             27.7ns ± 1%    26.3ns ± 0%   -5.12%  (p=0.008 n=5+5)
ContainsLower/long_line_no_match-10                 405ns ± 0%     249ns ± 1%  -38.36%  (p=0.008 n=5+5)
ContainsLower/long_line_match_start-10             7.04ns ± 0%    8.55ns ± 1%  +21.50%  (p=0.016 n=4+5)
ContainsLower/long_line_match_middle-10             181ns ± 0%     120ns ± 3%  -33.59%  (p=0.016 n=4+5)
ContainsLower/long_line_match_end-10                426ns ± 2%     272ns ± 0%  -36.05%  (p=0.008 n=5+5)
ContainsLower/short_unicode_line_no_match-10        178ns ± 0%      60ns ± 1%  -66.25%  (p=0.016 n=4+5)
ContainsLower/short_unicode_line_with_match-10     65.1ns ± 3%    37.9ns ± 0%  -41.80%  (p=0.016 n=5+4)
ContainsLower/long_unicode_line_no_match-10        1.98µs ± 1%    0.31µs ± 1%  -84.54%  (p=0.008 n=5+5)
ContainsLower/long_unicode_line_match_start-10      808ns ± 4%     243ns ± 0%  -69.94%  (p=0.016 n=5+4)
ContainsLower/long_unicode_line_match_middle-10     760ns ± 2%     108ns ± 3%  -85.77%  (p=0.008 n=5+5)
ContainsLower/long_unicode_line_match_end-10       3.89µs ± 1%    0.43µs ± 4%  -88.85%  (p=0.008 n=5+5)

*What this PR does / why we need it:

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

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

Sounds like test failure so will need to check