django opentracing: disconnected middleware spans with new django integration

[majorgreys]

When updating the https://github.com/DataDog/trace-examples/tree/master/python/opentracing/django example to the new Django integration (DataDog/dd-trace-py#1161), we gain visibility but the spans generated from the middleware are not part of the same trace.

Before update:

After update:

But middleware spans are detached from the django.request root:

[majorgreys]

Moved to DataDog/dd-trace-py#1198