enhancement(transforms): add rate_field and dynamic-field fallback support to sample

[jh7459-gh]

Summary

This PR improves the sample transform for per-event sampling use cases by adding dynamic field-driven sampling while preserving safe fallback behavior.

Changes included:

• Add ratio_field support for per-event ratio sampling ((0, 1]).
• Add rate_field support for per-event 1/N sampling (positive integer).
• Requires static sampling configuration (rate or ratio), as a default value. i.e. dynamic-only mode is not allowed.
• Disallow configuring ratio_field and rate_field together (mirrors the existing rate vs ratio exclusivity pattern).
• Keeps sample_rate_key populated with the effective sampling value used for each sampled event.
• Add validation and transform tests for fallback behavior and invalid configs.
• Update generated transform reference metadata and changelog fragment.

Vector configuration

[transforms.sample_dynamic]
type = "sample"
inputs = ["in"]

# required static fallback (exactly one of these)
ratio = 0.01
# rate = 100

# optional dynamic field (choose one)
ratio_field = "sample_ratio"
# rate_field = "sample_rate_n"

# optional deterministic/keyed sampling controls
key_field = "trace_id"
group_by = "{{ service }}"

How did you test this PR?

Ran locally:

• cargo fmt --all
• cargo test --no-default-features --features transforms-sample sample:: --lib
• cargo clippy --no-default-features --features transforms-sample --lib --tests -- -D warnings
• ./scripts/check_changelog_fragments.sh

Test coverage added/updated includes:

• reject dynamic ratio-only config
• reject dynamic rate-only config
• reject config that sets both dynamic fields
• accept static + dynamic config
• dynamic ratio overrides static
• dynamic rate overrides static
• dynamic ratio fallback to static ratio/rate when missing
• dynamic rate fallback to static ratio when missing

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

• Related: dynamic per-event sampling use case for sample transform
• Supersedes: enhancement(transforms): dynamic rate for sample #24976

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[jh7459-gh]

I have read the CLA Document and I hereby sign the CLA

[jh7459-gh]

recheck

[jh7459-gh]

Closing in favor of #24976 (reopened), where CLA and required lightweight checks are currently passing on the existing head SHA.