Provide a public Django middleware class

Is your feature request related to a problem?
We would to have control over what position django middleware gets placed in the chain. Currently the instrument() method inserts it at the head. In our case we would like to have exception tracking middleware (Rollbar) execute before tracing middleware.

Describe the solution you'd like
A public Django middleware class that can be configured in settings like so:

MIDDLEWARE = (
  "middleware.ExceptionTrackingMiddleware", 
  "opentelemetry.instrumentation.django.DjangoMiddleware",
)

Describe alternatives you've considered

• Modifying middleware chain at runtime (this seems dangerous)
• Wrapping the private middleware class (also seems dangerous)

Start by tracing the instrument() method and the private Django middleware class, then compare their behavior with Django's MIDDLEWARE settings. The work is done when a public opentelemetry.instrumentation.django.DjangoMiddleware can be configured in the middleware chain and allows exception tracking middleware to run before tracing middleware.