feat: support log line content filtering via RELABEL_CONFIGS

[Omerlustig]

Summary

• Enable RELABEL_CONFIGS to filter log entries based on log line content using a synthetic __log_line__ label, allowing users to drop noisy lines (e.g. END RequestId:) before they are sent to Loki
• The __log_line__ label is injected temporarily before relabeling and stripped afterwards so it never reaches Loki
• Fix a pre-existing bug where ScratchBuilder.Sort() was missing before relabel.Process, which uses binary search internally and produced non-deterministic results on unsorted labels

How it works

Users can now use the existing RELABEL_CONFIGS env var with source_labels: ["__log_line__"] to match against log line content:

[{"source_labels":["__log_line__"],"regex":"END RequestId:.*","action":"drop"}]

All Prometheus relabel actions (drop, keep, replace, labeldrop, etc.) are supported. Existing label-only relabel configs continue to work unchanged.

Test plan

• TestBatchAddDropsLogLinesViaRelabelConfig — drop action on log line content
• TestBatchAddKeepsEntriesWithNoRelabelConfig — no-op when no configs set
• TestBatchAddKeepActionFiltersLogLines — keep action filtering
• TestBatchAddLogLineLabelNotInOutput — __log_line__ label never in output
• TestBatchAddLabelOnlyRelabelStillWorks — backward compat with label-only relabel
• TestBatchAddDropMultiplePatterns — multi-pattern drop
• All 39 existing tests continue to pass

🤖 Generated with Claude Code

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