Consolidate Starlette/FastAPI middleware spans into one

[alexander-alderman-webb]

Currently, middleware spans are tracked generated per middleware layer. Create one span for the whole middleware instead.

The new span, encompassing all middleware layers, will be the default. Users can opt in to the old behavior of one span per layer.

We discussed that this could be done in a minor version because no one filters on middleware spans.

[linear]

PY-1978