dev-docs: Add smoke tests docs to TESTING.md

[marclop]

Motivation/summary

Adds a section about "smoke testing" in the TESTING.md.

Checklist

- [ ] Update CHANGELOG.asciidoc
- [ ] Update package changelog.yml (only if changes to apmpackage have been made)

• Documentation has been updated

How to test these changes

N/A

Related issues

Part of #8303

[apmmachine]

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Build stats

• Start Time: 2022-07-12T07:22:24.424+0000

• Duration: 27 min 25 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4108
Skipped	13
Total	4121

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[apmmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (42/42)	💚
Files	91.96% (183/199)	👍
Classes	93.435% (427/457)	👍
Methods	89.163% (1086/1218)	👍 0.082
Lines	76.782% (13314/17340)	👎 -0.029
Conditionals	100.0% (0/0)	💚

[mergify]

This pull request is now in conflicts. Could you fix it @marclop? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b f/add-smoke-testing-brief-documentation upstream/f/add-smoke-testing-brief-documentation
git merge upstream/main
git push upstream f/add-smoke-testing-brief-documentation

[apmmachine]

📚 Go benchmark report

Diff with the main branch

name                                                                                            old time/op    new time/op    delta
pkg:github.com/elastic/apm-server/agentcfg goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/decoder goos:linux goarch:amd64
CompressedRequestReader/deflate_content_encoding-12                                               16.8µs ± 1%    17.0µs ± 1%   +1.44%  (p=0.016 n=5+4)
pkg:github.com/elastic/apm-server/model/modelindexer goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/processor/stream goos:linux goarch:amd64
BackendProcessor/minimal.ndjson-12                                                                63.7µs ±14%    46.4µs ±38%  -27.10%  (p=0.016 n=5+5)
BackendProcessor/otel-bridge.ndjson-12                                                            34.1µs ±13%    29.1µs ±15%  -14.65%  (p=0.032 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel0/invalid-event-type.ndjson-12           790ns ± 1%     810ns ± 2%   +2.52%  (p=0.008 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel0/unknown-span-type.ndjson-12           6.54µs ± 1%    6.63µs ± 1%   +1.32%  (p=0.008 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel2/ratelimit.ndjson-12                   59.3µs ±12%    50.2µs ±11%  -15.43%  (p=0.032 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel2/transactions-huge_traces.ndjson-12    20.3µs ±17%    16.3µs ± 3%  -19.67%  (p=0.008 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel2/transactions.ndjson-12                56.1µs ±23%    44.7µs ±19%  -20.36%  (p=0.032 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel4/invalid-json-event.ndjson-12          3.08µs ±11%    3.40µs ± 7%  +10.27%  (p=0.032 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel8/invalid-json-event.ndjson-12          1.66µs ± 0%    1.68µs ± 1%   +1.78%  (p=0.016 n=4+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel8/span-links.ndjson-12                  2.46µs ± 3%    2.71µs ± 2%  +10.15%  (p=0.008 n=5+5)
pkg:github.com/elastic/apm-server/publish goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/aggregation/spanmetrics goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/aggregation/txmetrics goos:linux goarch:amd64
AggregateTransaction-12                                                                           72.4ns ± 1%    70.9ns ± 1%   -2.10%  (p=0.008 n=5+5)
pkg:github.com/elastic/apm-server/x-pack/apm-server/sampling goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/sampling/eventstorage goos:linux goarch:amd64
ReadEvents/json_codec/100_events-12                                                                664µs ± 0%     675µs ± 3%   +1.70%  (p=0.032 n=5+5)

name                                                                                            old alloc/op   new alloc/op   delta
pkg:github.com/elastic/apm-server/agentcfg goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/decoder goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/model/modelindexer goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/processor/stream goos:linux goarch:amd64
BackendProcessor/invalid-metadata.ndjson-12                                                       2.94kB ± 2%    2.87kB ± 2%   -2.24%  (p=0.040 n=5+5)
BackendProcessor/transactions.ndjson-12                                                           59.3kB ± 1%    59.9kB ± 1%   +0.96%  (p=0.032 n=5+5)
BackendProcessor/unknown-span-type.ndjson-12                                                      26.4kB ± 1%    26.8kB ± 1%   +1.71%  (p=0.016 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel0/invalid-event.ndjson-12               10.3kB ± 1%    10.4kB ± 1%   +0.95%  (p=0.016 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel0/invalid-metadata-2.ndjson-12          3.58kB ± 2%    3.54kB ± 1%   -1.30%  (p=0.040 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel2/invalid-json-metadata.ndjson-12       5.83kB ± 3%    5.71kB ± 2%   -2.19%  (p=0.040 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel2/minimal-service.ndjson-12             12.0kB ± 0%    11.8kB ± 1%   -1.35%  (p=0.008 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel2/transactions_spans_rum_2.ndjson-12    12.9kB ± 1%    12.8kB ± 1%   -0.99%  (p=0.032 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel4/errors_2.ndjson-12                    39.3kB ± 1%    39.7kB ± 1%   +0.89%  (p=0.032 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel8/transactions_spans.ndjson-12          57.6kB ± 0%    58.0kB ± 1%   +0.61%  (p=0.032 n=5+5)
pkg:github.com/elastic/apm-server/publish goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/aggregation/spanmetrics goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/aggregation/txmetrics goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/sampling goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/sampling/eventstorage goos:linux goarch:amd64

name                                                                                            old allocs/op  new allocs/op  delta
pkg:github.com/elastic/apm-server/agentcfg goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/decoder goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/model/modelindexer goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/processor/stream goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/publish goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/aggregation/spanmetrics goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/aggregation/txmetrics goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/sampling goos:linux goarch:amd64
pkg:github.com/elastic/apm-server/x-pack/apm-server/sampling/eventstorage goos:linux goarch:amd64

name                                                                                            old speed      new speed      delta
pkg:github.com/elastic/apm-server/processor/stream goos:linux goarch:amd64
BackendProcessor/minimal.ndjson-12                                                              16.4MB/s ±15%  23.6MB/s ±53%  +43.89%  (p=0.016 n=5+5)
BackendProcessor/otel-bridge.ndjson-12                                                          55.6MB/s ±12%  65.2MB/s ±14%  +17.28%  (p=0.032 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel0/invalid-event-type.ndjson-12         495MB/s ± 1%   482MB/s ± 2%   -2.46%  (p=0.008 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel0/unknown-span-type.ndjson-12          505MB/s ± 1%   499MB/s ± 1%   -1.30%  (p=0.008 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel2/ratelimit.ndjson-12                 71.6MB/s ±13%  84.2MB/s ±10%  +17.64%  (p=0.032 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel2/transactions-huge_traces.ndjson-12   158MB/s ±18%   195MB/s ± 3%  +22.96%  (p=0.008 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel2/transactions.ndjson-12               102MB/s ±21%   128MB/s ±17%  +24.53%  (p=0.032 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel4/invalid-json-event.ndjson-12         192MB/s ±12%   173MB/s ± 6%   -9.60%  (p=0.032 n=5+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel8/invalid-json-event.ndjson-12         355MB/s ± 0%   349MB/s ± 1%   -1.74%  (p=0.016 n=4+5)
BackendProcessorParallel/BenchmarkBackendProcessorParallel8/span-links.ndjson-12                 277MB/s ± 3%   252MB/s ± 2%   -9.23%  (p=0.008 n=5+5)

report generated with https://pkg.go.dev/golang.org/x/perf/cmd/benchstat