Support Metadata Plugins

[matt-hobbs-prima]

This PR introduces a plugin mechanism to allow integrators to specify / create their our metadata.

The plugins are passed as a setup option (metadata_plugins) and get a callback with the Absinthe.Blueprint to extract any information they need

[MaeIsBad]

No notes about the code, but

• can you update the changelog?
• what's the use case? What extra annotations would you want to add?

How about an alternative implementation where clients can attach metadata to the absinthe context instead of having this plugin system?

[matt-hobbs-prima]

No notes about the code, but

• can you update the changelog?
  > Yep, sorry missed it.
• what's the use case? What extra annotations would you want to add?
  > The use case for is that we want to know who's making GraphQL queries to Peano, alongside the metrics that are already gathered. (Trying to give it some love :) ) Some of the queries are anonymous so we want to add 'user-agent' as metadata. That concept doesn't exist in Absinthe so we have take the approach to add it to the context and then have a plugin that takes it from the context and adds it as metadata. Here is an implementation of how it would work.

How about an alternative implementation where clients can attach metadata to the absinthe context instead of having this plugin system?
 > That's an interesting (and simpler!) idea, let me experiment with it.

Thanks for the feedback

[Johnabell]

👍 Nice work would be good to update the changes log

[matt-hobbs-prima]

Closing this one in favour of:
#155