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?

Commencez par lire la classe Context et les chemins de fermeture et d’envoi de segment/subsegment, puis examinez comment le cas d’utilisation Aiohttp fournit le request context. Définissez l’API convenue pour joindre des annotations ou des métadonnées dérivées du contexte, notamment le moment où elles sont capturées et la manière dont les intégrations les utilisent. Le travail est terminé lorsque la conception est arrêtée et que le comportement est couvert pour les cas d’intégration indiqués.

Rédigé par le modèle d'indexation à partir du texte de l'issue.