feat(metrics): add Rust no_std encoder for V3 - #504
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Add a hand-rolled Rust encoder for metrics V3 payload format. For performance and compatibility reasons we don't want to rely on a generated encoder (which also vendors the protobuf dependency), so we add the hand-rolled version and guardrails to ensure that we can catch changes in the `.proto` definition. Signed-off-by: Mark Kirichenko <mark.kirichenko@datadoghq.com>
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The V3 writer interned each metric's tag ids in caller-supplied order but only sorted the emitted dictionary entry. The same logical tag set given in different orders (e.g. `["a","b"]` vs `["b","a"]`) therefore produced distinct `tagsetRefs` and duplicate `dictTagsets` entries instead of deduplicating, inflating payloads for producers with unstable tag ordering. Sort `tag_ids` before interning so the interner key matches the canonical on-wire (sorted) form; drop the now-redundant sort in `encode_tagset`. For a unique tag set the output is byte-identical; reordered duplicates now collapse to one ref and one dictionary entry. Add a unit test covering order-independent dedup, and criterion benchmarks (`benches/tagset.rs`) quantifying the added sort's cost on the cache-hit path (low-single-digit-percent, near-zero for typical tag counts). Signed-off-by: Mark Kirichenko <mark.kirichenko@datadoghq.com> Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> Signed-off-by: Mark Kirichenko <mark.kirichenko@datadoghq.com>
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What does this PR do?
Add a hand-rolled Rust encoder for metrics V3 payload format.
Motivation
For performance and compatibility reasons we don't want to rely on a generated encoder (which also vendors the protobuf dependency), so we add the hand-rolled version and guardrails to ensure that we can catch changes in the
.protodefinition.Describe how to test/QA your changes
This commit adds tests which compare wire payloads created by this encoder vs payloads created by a generated encoder - those must match.
Reviewer's Checklist
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