Document output.elasticsearch.api_key in yml file

[simitt]

related to #3137

[codecov-io]

Codecov Report

Merging #3138 into master will decrease coverage by 0.73%.
The diff coverage is 84.26%.

@@            Coverage Diff             @@
##           master    #3138      +/-   ##
==========================================
- Coverage   79.63%   78.89%   -0.74%     
==========================================
  Files         105      106       +1     
  Lines        5529     5587      +58     
==========================================
+ Hits         4403     4408       +5     
- Misses       1126     1179      +53

Impacted Files	Coverage Δ
elasticsearch/client.go	27.69% <ø> (-2.31%)	⬇️
sourcemap/es_store.go	85.89% <ø> (ø)	⬆️
beater/config/instrumentation.go	17.85% <ø> (ø)	⬆️
elasticsearch/security_api.go	0% <ø> (ø)
beater/config/api_key.go	85.71% <ø> (-1.79%)	⬇️
elasticsearch/config.go	72.91% <ø> (ø)	⬆️
beater/beater.go	64.28% <ø> (-1.79%)	⬇️
beater/tracing.go	78.26% <100%> (+0.73%)	⬆️
beater/api/root/handler.go	88.23% <100%> (ø)	⬆️
beater/authorization/privilege.go	100% <100%> (ø)	⬆️
... and 11 more

[graphaelli]

if supported, we should have this in all of the elasticsearch sections - monitoring_elasticsearch, source_mapping

[simitt]

Good point!

• monitoring.elasticsearch was also updated in libbeat, so it is supposed to be supported.
• apm-server.sourcemap automatically supports it via https://github.com/elastic/apm-server/blob/master/elasticsearch/config.go#L56.
• I didn't add it to apm-server.api_key - theoretically an api_key could be used to create api_keys, but I think documenting that would be more confusing than helping.

[graphaelli]

is this the base64 encoded version or really two strings with colon?

[simitt]

It is really two strings with colon, the libbeat elasticsearch client takes care of the base64 encoding.
We should be consistent with this behaviour also for sourcemaps, so I am changing the elasticsearch/client behavior accordingly.
Unfortunately we currently do not have system tests for the case that an apm-server.source_mapping.elasticsearch is configured, created an issue for following up on that #3166.

[graphaelli]

great, thanks for clarifying

[graphaelli]

great, thanks for clarifying