Exclude agent plugins from instrumentation

[SylvainJuge]

This is a follow-up issue of #2724, where the issue was excluding the ECS logging instrumentation for the internal copy of ECS logging within the agent (thus loaded within agent/internal plugin classloader), but not the one that might be present in the application.

Classes that are loaded by the internal plugin classloader do not need to be instrumented and should thus be excluded from instrumentation.

See #2724 (comment) for hints on why this might be required.

For external plugins, instrumentation is required in order to allow the agent to delegate to internal tracer.

Few ideas that needs to be explored:

• can we exclude by default for all instrumentations without having to rewrite all overrides of co.elastic.apm.agent.sdk.ElasticApmInstrumentation#getClassLoaderMatcher
• can we provide an opt-in exclusion from this exclusion, which could be for example used for external OpenTelemetry or for the rare cases that require agent self-instrumentation ?

[jackshirazi]

maybe worth having that on a property, there's a scenario where a customer wants to use an external plugin to adjust internally loaded instrumentation