agent_config metrics are not being indexed

[axw]

APM Server version (apm-server version): 8.5.0-SNAPSHOT (main)

Description of the problem including expected versus actual behavior:

agent_config metrics are not being indexed - they are failing to be indexed due to an authorization error. This is most likely related to the refactoring I recently did around setting up model processors; it looks like the agentcfg reporter is attempting to send metrics to the pre-processing chain, which requires authorization.

Also, we don't have an system tests that verify the metrics are indexed.

Steps to reproduce:

1. Run the TestAgentConfig system test
2. Observe that no "agent_config" metric is ever indexed

Provide logs (if relevant):

{"log.level":"error","@timestamp":"2022-09-06T04:40:45.565Z","log.logger":"agentcfg","log.origin":{"file.name":"agentcfg/reporter.go","file.line":106},"message":"error sending applied agent configs to kibana: no Authorizer in context","service.name":"apm-server","ecs.version":"1.6.0"}

[simitt]

We weren't using the metrics anywhere. Are they even still relevant?

[axw]

@simitt the UI started using the metrics in 8.3: elastic/kibana#123109