Warn of presence of multiple APM Agents

[lucabelluccini]

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.

[jackshirazi]

We can

• detect classes loaded by the other agents for known agents (eg ElasticApmAgent class already looks for these to specifically avoid instrumenting them)
• look at the command-line for other -javaagent and -agentlib
• ?

[SylvainJuge]

I think this is definitely a great idea, but it could be trickly to implement properly.

Parsing -javaagent could produce false positives as some application servers are shipped with an agent by default.

Adding an heuristic to detect other agents would be doable, but would likely require to implement and maintain a "list of known agents", trying to open other -javaagent jar files and detect on the META-INF/MANIFEST.MF entries could be a good start.