Tell APM Server the location of ECS-JSON log files.

A follow-up on #373

Once the agent has re-configured the application to write ECS-JSON logs, it should send the location of the log files to APM Server. This should happen if and only if APM Server is running on localhost.

APM Server will then act as a local provider to tell Filebeat to automatically ship these log files.

If the application doesn't log to files bu to stdout, for example, then there's nothing to send to APM Server

Start by reading the related #373 work and trace how the agent reconfigures application logging. Determine where a localhost APM Server integration can receive ECS-JSON log-file locations, while stdout-only logging sends nothing. Done means the location is sent only when APM Server is local and file logging is configured.

Written by the indexing model from the issue text.