Update compute_stats_by_span_kind documentation

[liustanley]

What does this PR do?

Updates documentation for compute_stats_by_span_kind based on changes from #22163. Also adds clarification in previous release note to address similar config options.

Motivation

Clarification on top-level spans update.

Additional Notes

Possible Drawbacks / Trade-offs

Describe how to test/QA your changes

[pr-commenter]

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=30550328 --os-family=ubuntu

[pr-commenter]

Regression Detector

Regression Detector Results

Run ID: 48580cb9-dc70-4c16-896b-87ada22cdc9e
Baseline: 4177771
Comparison: 3b821bf

Performance changes are noted in the perf column of each table:

• ✅ = significantly better comparison variant performance
• ❌ = significantly worse comparison variant performance
• ➖ = no significant change in performance

No significant changes in experiment optimization goals

Confidence level: 90.00%
Effect size tolerance: |Δ mean %| ≥ 5.00%

There were no significant changes in experiment optimization goals at this confidence level and effect size tolerance.

Experiments ignored for regressions

Regressions in experiments with settings containing erratic: true are ignored.

perf	experiment	goal	Δ mean %	Δ mean % CI
➖	file_to_blackhole	% cpu utilization	+2.58	[-3.73, +8.89]

Fine details of change detection per experiment

perf	experiment	goal	Δ mean %	Δ mean % CI
➖	file_to_blackhole	% cpu utilization	+2.58	[-3.73, +8.89]
➖	pycheck_1000_100byte_tags	% cpu utilization	+2.30	[-2.63, +7.23]
➖	idle	memory utilization	+1.27	[+1.23, +1.31]
➖	basic_py_check	% cpu utilization	+0.84	[-1.59, +3.26]
➖	trace_agent_json	ingress throughput	+0.03	[-0.02, +0.07]
➖	trace_agent_msgpack	ingress throughput	+0.00	[-0.00, +0.00]
➖	uds_dogstatsd_to_api	ingress throughput	-0.00	[-0.20, +0.20]
➖	tcp_dd_logs_filter_exclude	ingress throughput	-0.01	[-0.02, +0.01]
➖	otel_to_otel_logs	ingress throughput	-0.04	[-0.47, +0.40]
➖	file_tree	memory utilization	-0.23	[-0.34, -0.13]
➖	process_agent_standard_check	memory utilization	-0.35	[-0.40, -0.31]
➖	process_agent_standard_check_with_stats	memory utilization	-0.43	[-0.48, -0.39]
➖	tcp_syslog_to_blackhole	ingress throughput	-0.46	[-0.55, -0.37]
➖	process_agent_real_time_mode	memory utilization	-0.63	[-0.68, -0.59]
➖	uds_dogstatsd_to_api_cpu	% cpu utilization	-1.62	[-4.34, +1.10]

Explanation

A regression test is an A/B test of target performance in a repeatable rig, where "performance" is measured as "comparison variant minus baseline variant" for an optimization goal (e.g., ingress throughput). Due to intrinsic variability in measuring that goal, we can only estimate its mean value for each experiment; we report uncertainty in that value as a 90.00% confidence interval denoted "Δ mean % CI".

For each experiment, we decide whether a change in performance is a "regression" -- a change worth investigating further -- if all of the following criteria are true:

1. Its estimated |Δ mean %| ≥ 5.00%, indicating the change is big enough to merit a closer look.

2. Its 90.00% confidence interval "Δ mean % CI" does not contain zero, indicating that if our statistical model is accurate, there is at least a 90.00% chance there is a difference in performance between baseline and comparison variants.

3. Its configuration does not mark it "erratic".

[maycmlee]

Left a couple of suggestions!

[liustanley]

/merge

[dd-devflow]

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[dd-devflow]

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