refactor(sampling)!: use sampling from libdatadog [APMSP-3021]

[bantonsson]

Moves the sampling code into a separate crate, and uses the moved sampling code from libdatadog DataDog/libdatadog#1927

These are the benchmark numbers after DataDog/libdatadog#1977 landed.

OTel Sampling Benchmarks vs main

Allocations

Benchmark	Allocated	Change
rule_all_spans_only_rate	0.732 KB	±0.0% (no change)
service_rule_matching	0.756 KB	-92.1% 🟢
service_rule_not_matching	0.434 KB	-2.9% 🟢
name_pattern_rule_matching	0.746 KB	-92.2% 🟢
name_pattern_rule_not_matching	0.460 KB	-7.8% 🟢
resource_pattern_rule_matching	0.502 KB	-7.2% 🟢
resource_pattern_rule_not_matching	0.500 KB	-6.7% 🟢
tag_rule_matching	1.021 KB	-89.9% 🟢
tag_rule_not_matching	0.460 KB	-6.5% 🟢
complex_rule_matching	0.477 KB	-9.1% 🟢
complex_rule_partial_match	0.470 KB	-8.2% 🟢
multiple_rules_first_match	0.756 KB	-92.1% 🟢
multiple_rules_last_match	0.758 KB	-3.3% 🟢
many_attributes	1.012 KB	-90.0% 🟢
parent_sampled_short_circuit	~0 B	no change
parent_not_sampled_short_circuit	~0 B	no change
unicode_rule_matching	0.496 KB	±0.0% (no change)

Wall Time

Benchmark	Time	Change
rule_all_spans_only_rate	329 ns	-2.9% 🟢
service_rule_matching	461 ns	-14.8% 🟢
service_rule_not_matching	244 ns	-7.6% 🟢
name_pattern_rule_matching	363 ns	-18.0% 🟢
name_pattern_rule_not_matching	177 ns	-14.1% 🟢
resource_pattern_rule_matching	252 ns	-11.0% 🟢
resource_pattern_rule_not_matching	256 ns	-10.7% 🟢
tag_rule_matching	386 ns	-17.5% 🟢
tag_rule_not_matching	204 ns	-13.8% 🟢
complex_rule_matching	386 ns	-10.9% 🟢
complex_rule_partial_match	385 ns	-11.1% 🟢
multiple_rules_first_match	376 ns	-17.6% 🟢
multiple_rules_last_match	510 ns	-6.8% 🟢
many_attributes	392 ns	-16.9% 🟢
parent_sampled_short_circuit	9.2 ns	~-0.8% (noise)
parent_not_sampled_short_circuit	9.2 ns	~-0.7% (noise)
unicode_rule_matching	251 ns	-1.8% 🟢

[ekump]

a couple of non-blocking comments. LGTM

[datadog-prod-us1-6]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 0a4e479 | Docs | Datadog PR Page | Give us feedback!}

[bantonsson]

/merge

[gh-worker-devflow-routing-ef8351]

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2026-05-20 14:16:45 UTC ℹ️ Start processing command /merge

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[iunanua]

To prevent crates with unsupported msrv from slipping in, could we define in .cargo/config.toml something like

[resolver]
incompatible-rust-versions = "fallback"

?

[bantonsson]

@iunanua I have no idea. If you know that incompatible-rust-versions = "fallback" is a good idea, please say so. There are too many knobs and levers on cargo.