Add ability to have middleware in tracers

It would be nice to have a way to add a middleware in the tracers.

This would allow for say adding extra attributes at the top level request.

Look at the existing tracer implementations in the repository to understand the current structure. Identify where middleware could be inserted, likely in the tracer's request handling flow. Check for similar patterns in the main gqlgen library or other Go tracing libraries. The goal is to allow adding extra attributes at the top level of a request.

索引モデルが issue の本文から書いたものです。