Support for generic annotations and meta for third-party libraries

Hi, we would need to send some annotations (or meta) data along with the data that is sent automatically as a segment or subsegment. Taking into account that this data is grabbed from the context, so this data changes at each request. A good example is the `request_id` that is progressed as a header between all of the services.

We are keen on the Aiohttp user case but might be worth it have a solution that fits all of the integrations.

A possible solution that wouldn't introduce much technical debt would be adding a new method of the `Context` that would be called each time that a segment is closed/sent to add all extra meta/annotations. By default will do nothing, but the developer would have the chance to override this method subclassing the `Context` class and implementing its own logic.

Thoughts, any other ideas?