refactor(loki): Add internal sampling library for secretfilter and process stage

[mikefat]

Brief description of Pull Request

Based on a comment here, this PR refactors the code used for rate-based sampling and organizes it into an internal shared library. This cuts on code duplication and creates one source of truth for logic. This is currently used by loki.secretfilter and loki.process stage.sampling.

Pull Request Details

Changes:

• internal/sampling: New package with Sampler struct. Implements the same probabilistic sampling as before (Jaeger-style algorithm) using stdlib only.
• loki.secretfilter: Replaces local sampling (boundary/source, shouldProcessEntry) with *sampling.Sampler; Validate uses sampling.ValidateRate.
• loki.process stage.sampling: Replaces local sampling and the jaeger-client-go dependency with *sampling.Sampler; config validation uses sampling.ValidateRate. Rate remains a required attribute (no default).
• Tests: internal/sampling has new unit tests

Notes to the Reviewer

PR Checklist

• Tests updated

[github-actions]

🔍 Dependency Review

github.com/uber/jaeger-client-go v2.30.0+incompatible (direct) → v2.30.0+incompatible (indirect) — ✅ Safe

• Summary

  • Version is unchanged; only the requirement type changed from direct to indirect.
  • No API or behavior change to review across versions; no code changes are required to adopt this state.

• Changelog / Release Notes

  • No version delta to review (as-is and to-be are identical: v2.30.0+incompatible).
  • Reference: https://github.com/uber/jaeger-client-go/releases/tag/v2.30.0

• Suggested Code Changes

  • None required. Because the version did not change and it’s now an indirect dependency, there are no code updates needed.

Notes

• No net-new dependencies were introduced in go.mod in this diff.

[kalleep]

LGTM, thanks!