beater: telemetry for tail-based sampling config

[axw]

Motivation/summary

Add some basic telemetry for tail-sampling config: if it's enabled, and the number of policies defined.

Checklist

• I have signed the Contributor License Agreement.
• I have updated CHANGELOG.asciidoc

I have considered changes for:
- [ ] documentation
- [ ] logging (add log lines, choose appropriate log selector, etc.)
- [ ] metrics and monitoring (create issue for Kibana team to add metrics to visualizations, e.g. Kibana#44001)

• automated tests (add tests for the code changes, all unit tests pass locally)
• telemetry
• Elasticsearch Service (https://cloud.elastic.co)
• Elastic Cloud Enterprise (https://www.elastic.co/products/ece)
• Elastic Cloud on Kubernetes (https://www.elastic.co/elastic-cloud-kubernetes)

How to test these changes

1. Run apm-server with monitoring enabled, and without tail-sampling enabled: you should see apm-server.sampling.tail.enabled: false in a beats_state monitoring doc.
2. Run apm-server with monitoring enabled, and tail-sampling enabled with two policies: you should see apm-server.sampling.tail.enabled: true and apm-server.sampling.tail.policies: 2 in a beats_state monitoring doc.

Related issues

#4185

[apmmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Build Cause: [Pull request #4360 updated]

• Start Time: 2020-10-29T01:05:55.533+0000

• Duration: 56 min 45 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4376
Skipped	145
Total	4521

Steps errors

Expand to view the steps failures

• Name: Compress

  • Description: tar --exclude=coverage-files.tgz -czf coverage-files.tgz coverage

  • Duration: 0 min 0 sec

  • Start Time: 2020-10-29T01:22:36.464+0000

  • log

• Name: Compress

  • Description: tar --exclude=system-tests-linux-files.tgz -czf system-tests-linux-files.tgz system-tests

  • Duration: 0 min 0 sec

  • Start Time: 2020-10-29T01:36:05.010+0000

  • log

[codecov-io]

Codecov Report

Merging #4360 into master will increase coverage by 0.03%.
The diff coverage is 79.61%.

@@            Coverage Diff             @@
##           master    #4360      +/-   ##
==========================================
+ Coverage   79.67%   79.71%   +0.03%     
==========================================
  Files         161      162       +1     
  Lines        8983     9173     +190     
==========================================
+ Hits         7157     7312     +155     
- Misses       1826     1861      +35     

Impacted Files	Coverage Δ
beater/beater.go	61.45% <0.00%> (-0.85%)	⬇️
beater/config/sampling.go	84.61% <ø> (-0.57%)	⬇️
model/modeldecoder/v2/model.go	100.00% <ø> (ø)
x-pack/apm-server/main.go	0.00% <0.00%> (ø)
beater/telemetry.go	83.33% <50.00%> (-1.67%)	⬇️
sourcemap/mapper.go	68.18% <63.15%> (-31.82%)	⬇️
model/modeldecoder/v2/model_generated.go	70.40% <71.95%> (-0.01%)	⬇️
processor/stream/processor.go	75.63% <77.77%> (-0.90%)	⬇️
x-pack/apm-server/sampling/processor.go	79.62% <89.09%> (+3.50%)	⬆️
x-pack/apm-server/sampling/groups.go	93.00% <91.35%> (+6.43%)	⬆️
... and 12 more

[simitt]

Tested with BC1 - works as expected