Make sanitize_field_names centrally configurable

[axw]

Closes #821

[codecov-io]

Codecov Report

Merging #856 (f4a273c) into master (d72ef83) will decrease coverage by 0.11%.
The diff coverage is 86.36%.

@@            Coverage Diff             @@
##           master     #856      +/-   ##
==========================================
- Coverage   83.83%   83.72%   -0.12%     
==========================================
  Files         141      141              
  Lines        7849     7863      +14     
==========================================
+ Hits         6580     6583       +3     
- Misses        863      871       +8     
- Partials      406      409       +3     

Impacted Files	Coverage Δ
apmtest/configwatcher.go	100.00% <ø> (ø)
apmtest/discard.go	80.00% <ø> (ø)
apmtest/env.go	100.00% <ø> (ø)
apmtest/httpsuite.go	100.00% <ø> (ø)
apmtest/recorder.go	77.77% <ø> (ø)
apmtest/recordlogger.go	87.50% <ø> (ø)
apmtest/testlogger.go	66.66% <ø> (ø)
apmtest/withtransaction.go	100.00% <ø> (ø)
breakdown.go	95.38% <ø> (ø)
builtin_metrics.go	81.81% <ø> (ø)
... and 104 more

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[apmmachine]

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Build stats

• Build Cause: Pull request #856 updated

• Start Time: 2020-12-08T09:14:23.603+0000

• Duration: 26 min 1 sec

Test stats 🧪

Test	Results
Failed	0
Passed	7648
Skipped	185
Total	7833

[AlexanderWert]

@axw Thanks for finishing this! Could you also please add the Go agent in Kibana for this central config option: https://github.com/elastic/kibana/blob/caa14d44efedc801fd1f779e15442ca9d5f2fab7/x-pack/plugins/apm/common/agent_configuration/setting_definitions/general_settings.ts#L238

[axw]

Thanks for the reminder @AlexanderWert, I've opened elastic/kibana#85373