v2: remove deprecated samplingpriority

[hannahkm]

What does this PR do?

Remove support for the SamplingPriority constant, which was deprecated in PR #1082. Replace instances with ManualKeep and ManualDrop.

• Previously, "kept" spans and traces could have a SamplingPriority of either 1 or 2. They should now use samplingDecision = 2 to indicate they should be kept.
• "dropped" spans and traces used to have SamplingPriority 0 or -1. Now, it has value samplingDecision=1

Motivation

For v2 release, we should remove support for deprecated constants and functions, especially those that have been deprecated for many, subsequent minor versions.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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[pr-commenter]

Benchmarks

Benchmark execution time: 2024-09-26 14:54:22

Comparing candidate commit 301391b in PR branch hannah/remove-samplingpriority with baseline commit 7d4b68c in branch v2-dev.

Found 2 performance improvements and 12 performance regressions! Performance is the same for 39 metrics, 0 unstable metrics.

scenario:BenchmarkInjectW3C-24

• 🟩 execution_time [-136.856ns; -105.544ns] or [-3.245%; -2.502%]

scenario:BenchmarkSetTagMetric-24

• 🟩 execution_time [-6.234ns; -3.546ns] or [-5.193%; -2.954%]

scenario:BenchmarkSingleSpanRetention/no-rules-24

• 🟥 allocated_mem [+169.626KB; +169.912KB] or [+102.420%; +102.593%]
• 🟥 allocations [+1724; +1724] or [+120.982%; +120.982%]
• 🟥 execution_time [+42.376µs; +44.097µs] or [+16.927%; +17.615%]

scenario:BenchmarkSingleSpanRetention/with-rules/match-all-24

• 🟥 allocated_mem [+166.887KB; +167.128KB] or [+99.209%; +99.352%]
• 🟥 allocations [+1724; +1724] or [+120.982%; +120.982%]
• 🟥 execution_time [+45.054µs; +47.613µs] or [+17.890%; +18.906%]

scenario:BenchmarkSingleSpanRetention/with-rules/match-half-24

• 🟥 allocated_mem [+165.930KB; +166.159KB] or [+98.695%; +98.831%]
• 🟥 allocations [+1724; +1724] or [+120.982%; +120.982%]
• 🟥 execution_time [+43.619µs; +44.812µs] or [+17.312%; +17.786%]

scenario:BenchmarkTracerAddSpans-24

• 🟥 allocated_mem [+2.211KB; +2.217KB] or [+94.278%; +94.541%]
• 🟥 allocations [+25; +25] or [+100.000%; +100.000%]
• 🟥 execution_time [+516.578ns; +552.622ns] or [+12.539%; +13.413%]