Document class loader implications of sharing instrumentation classes and multiple library dependencies

[trask]

This is a tricky area that has come up a couple of times recently. I think it would be helpful to document somewhere to make sure we have a shared understanding of the issue(s) and solution(s).

e.g.

#4236 (comment):

I think that it might be better to put KafkaClientsConsumerProcessTracing into a bootstrap module as when kafka-clients and kafka-streams are in different class loaders they would each get their own copy of that class. The same applies to KafkaConsumerIterableWrapper and KafkaConsumerIteratorWrapper that are used from other instrumentations, probably should have a KafkaWrapper interface in bootstrap module.

#3718 (comment):

My understanding is that doesn't really work when 2 modules are in different class loaders. Muzzle will fail on spring-web because instrumentation uses classes from spring-webmvc hence they can't be in the same InstrumentationModule.