Warn of presence of multiple APM Agents

## Is your feature request related to a problem?

We are aware multiple APM Agents might conflict each other https://www.elastic.co/guide/en/apm/agent/java/current/trouble-shooting.html#trouble-shooting-additional-agent
We need to detect it as soon as possible.

## Describe the solution you'd like

Attempt to detect if other "agents" are running together with Elastic APM Agent and log a warning at startup of the application.

## Describe alternatives you've considered

Alternative is to check this manually and it might be easily overlooked.