V2: Allow easily filtering only the agent logs from the combined agent log file

[cmacknz]

Comment originally made here #1702 (comment)

After #221 the spawned components contain message fields to easily allow separating them out when analyzing logs locally:

"component":{"binary":"metricbeat","dataset":"elastic_agent.http_metrics_monitoring","id":"http/metrics-monitoring","type":"http/metrics"},"log.origin":{"file.line":721,"file.name":"instance/beat.go"},"service.name":"metricbeat","ecs.version":"1.6.0","ecs.version":"1.6.0"}

For example we have component":{"binary":"metricbeat","dataset":"elastic_agent.http_metrics_monitoring","id":"http/metrics-monitoring","type":"http/metrics"} and "service.name":"metricbeat" to allow looking at the monitoring metricbeat logs only.

We are missing these with logs coming from the agent itself:

{"log.level":"info","@timestamp":"2022-11-28T19:59:12.045Z","log.origin":{"file.name":"coordinator/coordinator.go","file.line":645},"message":"Updating running component model","ecs.version":"1.6.0"}

We should add at least "service.name":"agent" to allow filtering for agent specific logs easily e.g. being able to do rg '"service.name":"agent"' locally will save a lot of time during SDH analysis. The agent itself isn't a component so it makes sense for those fields to be missing.

[michalpristas]

closing as #2044 went in