Programmatically disabling instrumentation on classpath

[breedx-splk]

This is a tracking issue that follows up on this comment in #408.

Now that we've moved to the fancy new instrumentation API, we have a case where all instrumentation that is included at build-time should show up on the classpath at runtime in order to be disabled. There may be cases, however, where users want to programmatically disable some instrumentation at runtime based on configuration or something else (a probabilistic model, whatever the case may be).

It's perfectly reasonable to defer this until we have an actual ask from an end-user, but I wanted to make sure it had been captured.