pkg/trace/api: create pool for traces

[knusbaum]

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What does this PR do?

The decoder regularly allocates space for traces and then discards it. Reducing this should reduce garbage and GC pressure.

Motivation

Additional Notes

For this one, I was not able to make a Go Benchmark work. Instead, I'll attach this image from a profile showing significant reduction in allocations from a local test:

Possible Drawbacks / Trade-offs

Describe how to test/QA your changes

[pr-commenter[bot]]

Benchmarks

Benchmark execution time: 2024-05-31 20:42:20

Comparing candidate commit c0a2592fc61ed3c85fbef8678d4de0979986d47f in PR branch knusbaum/pool-traces with baseline commit bba6f6d17eb5b0ca7549d63134f837533ce87562 in branch main.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 1 metrics, 1 unstable metrics.

scenario:BenchmarkAgentTraceProcessing-24

• 🟩 allocated_mem [-1.907MB; -1.680MB] or [-45.647%; -40.214%]

[knusbaum]

The profile in the description was not normalized, and it doesn't appear that there's a huge difference. Need to look into this more.

[pr-commenter[bot]]

Test changes on VM

Use this command from test-infra-definitions to manually test this PR changes on a VM:

inv create-vm --pipeline-id=35621324 --os-family=ubuntu

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 73.09942% with 46 lines in your changes missing coverage. Please review.

Project coverage is 62.27%. Comparing base (9f39b07) to head (c0a2592). Report is 1913 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/trace/writer/trace.go	73.91%	20 Missing and 4 partials :warning:
comp/trace/agent/agent_mock.go	0.00%	9 Missing :warning:
pkg/trace/agent/agent.go	66.66%	4 Missing :warning:
...mp/otelcol/otlp/components/statsprocessor/agent.go	62.50%	3 Missing :warning:
pkg/trace/writer/sender.go	78.57%	3 Missing :warning:
pkg/trace/stats/concentrator.go	50.00%	2 Missing :warning:
pkg/trace/writer/stats.go	87.50%	1 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #26205       +/-   ##
===========================================
+ Coverage   45.17%   62.27%   +17.09%     
===========================================
  Files        2346      242     -2104     
  Lines      270606    21085   -249521     
===========================================
- Hits       122246    13130   -109116     
+ Misses     138710     7394   -131316     
+ Partials     9650      561     -9089     

Flag	Coverage Δ
amzn_aarch64	62.64% <73.09%> (+16.64%)	:arrow_up:
centos_x86_64	62.66% <73.09%> (+16.74%)	:arrow_up:
ubuntu_aarch64	62.66% <73.09%> (+16.65%)	:arrow_up:
ubuntu_x86_64	62.69% <73.09%> (+16.69%)	:arrow_up:
windows_amd64	60.66% <73.09%> (+9.26%)	:arrow_up:

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