WIP: internal/traceprof: make endpoint call counting efficient

[nsrip-dd]

Switch to a data structure designed for efficient concurrent updates for
endpoint call counting. This is a prerequisite for enabling the feature
by default.

TODO:

• Say more about the choice of library. Is it well tested? Correct?
• Think harder about corner cases, especially around the size limit
  and concurrent modification when reading

Benchmark results on my M1 Max Macbook Pro:

goos: darwin
goarch: arm64
pkg: gopkg.in/DataDog/dd-trace-go.v1/internal/traceprof
cpu: Apple M1 Max
				 │   before.txt   │               after.txt               │
				 │     sec/op     │    sec/op      vs base                │
EndpointCounter/enabled=true-10    139.800n ± 11%    5.000n ± 18%  -96.42% (p=0.000 n=10)
EndpointCounter/enabled=false-10    0.2716n ±  5%   0.2682n ±  1%        ~ (p=0.118 n=10)
geomean                              6.162n          1.158n        -81.21%

				 │  before.txt  │              after.txt              │
				 │     B/op     │    B/op     vs base                 │
EndpointCounter/enabled=true-10    0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
EndpointCounter/enabled=false-10   0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
geomean                                       ²               +0.00%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean

				 │  before.txt  │              after.txt              │
				 │  allocs/op   │ allocs/op   vs base                 │
EndpointCounter/enabled=true-10    0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
EndpointCounter/enabled=false-10   0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
geomean                                       ²               +0.00%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-02-05 18:31:39

Comparing candidate commit 071ccfd in PR branch nick.ripley/fast-prof-endpoint-call-counting with baseline commit 739be25 in branch main.

Found 0 performance improvements and 2 performance regressions! Performance is the same for 57 metrics, 0 unstable metrics.

scenario:BenchmarkHttpServeTrace-24

• 🟥 execution_time [+336.269ns; +452.731ns] or [+2.098%; +2.825%]

scenario:BenchmarkSetTagStringPtr-24

• 🟥 execution_time [+4.396ns; +8.324ns] or [+2.533%; +4.796%]

[datadog-datadog-prod-us1]

Datadog Report

Branch report: nick.ripley/fast-prof-endpoint-call-counting
Commit report: 83474b9
Test service: dd-trace-go

✅ 0 Failed, 5222 Passed, 74 Skipped, 2m 14.34s Total Time