Simpler solution for library instrumentation to benefit from InstrumentationContext when used in agent

[trask]

If we had an instrumentation-api something like:

public AssociatingCache {
  public static <Q extends K, K, C> Cache<Q, C> get(Class<K> keyClass, Class<C> contextClass) {
    return Cache.newBuilder().setWeakKeys().build();
  }
}

then the agent could replace the implementation of that method at runtime with

InstrumentationContext.get(keyClass, contextClass).asCache()

and I think that would make it easier to migrate things to library instrumentation while not losing the benefit of InstrumentationContext when used in agent.

(see JdbcData.java for the current solution to using InstrumentationContext from library instrumentation when it is used in agent)

[mateuszrzeszutek]

We'd probably have to do sth like

if (contextStoreSupplier == null) {
  return Cache.newBuilder().setWeakKeys().build();
}
return contextStoreSupplier.get(keyClass, contextClass);

With the contextStoreSupplier being set by the agent.