Add service name to logs

[jalvz]

Implements part of https://github.com/elastic/apm-server/issues/5034

Checklist

• Update CHANGELOG.asciidoc

How to test these changes

Send some events to apm-server, check that service.name is in the request logs

[ghost]

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• Build Cause: Pull request #5355 updated

• Start Time: 2021-05-27T13:14:15.745+0000

• Duration: 38 min 34 sec

• Commit: 55c757d

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[axw]

Explicitly adding service.name to every log message is going to be difficult and error-prone to maintain. Moreover, we should be adding the fields to log messages coming from libbeat too.

The service name is the beat name, and that's already known to logp, so can we add the field centrally in logp? It should be possible to do this with a zap.Fields option, or with zap.Logger.WithFields.

Given that we'll also need to set event.dataset as part of https://github.com/elastic/apm-server/issues/5034, maybe we should add an option in ecszap for setting both service.name and event.dataset?

[jalvz]

The service name is the beat name

I understood it was the apm service name, not the beat name. The this doesn't make sense, closing.

[axw]

Tested changed made in elastic/beats#25998, using 7.14 BC1. Example apm-server log record:

localtesting_7.14.0_apm-server | {"log.level":"info","@timestamp":"2021-07-06T07:56:51.344Z","log.logger":"request","log.origin":{"file.name":"middleware/log_middleware.go","file.line":63},"message":"request ok","service.name":"apm-server","event.dataset":"apm-server","url.original":"/","http.request.method":"GET","user_agent.original":"curl/7.29.0","source.address":"127.0.0.1","http.request.body.bytes":0,"http.request.id":"bb9ec214-10f0-4c7a-95d9-1c48bcf50928","event.duration":97785,"http.response.status_code":200,"ecs.version":"1.6.0"}

Notably, service.name and event.dataset fields are there.