ref(tracing): Disable Django middleware span auto-instrumentation (#102290)

It has been very rare that we've needed to debug the performance of our
middleware spans, and these are a significant volume of our span
telemetry we send. Let's disable them and re-add telemetry in specific
places if required.

We'll also still have profiles which can be used to debug these
middlewares with full detail if needed.

Author: JoshFerge

src/sentry/utils/sdk.py

@@ -482,7 +482,7 @@ def flush(
         transport=MultiplexingTransport(),
         integrations=[
             DjangoAtomicIntegration(),
-            DjangoIntegration(signals_spans=False, cache_spans=True),
+            DjangoIntegration(signals_spans=False, cache_spans=True, middleware_spans=False),
             # This makes it so all levels of logging are recorded as breadcrumbs,
             # but none are captured as events (that's handled by the `internal`
             # logger defined in `server.py`, which ignores the levels set