Introduce 'instance augmentation' functionality

[geeknoid]

We've discussed previously in a few context the need to allow operators to augment instances with custom data, and in fact the general need to let operators mutate existing instances before dispatching them to adapters.

It should be possible for adapter-level configuration to take incoming instances and push custom state into the adapter alongside the instance (for example, by filling some canonical 'extensions' field on the instance).

For traces flowing though Mixer, we want to let operators augment the data as it flows through Mixer.

This issue is about tracking the design and implementation work of this feature.