✨(backend) allow configuring trace sampling

Add a configuration knob for the trace sampling rate, so we can
enable tracing on middleware and cache spans when debugging slow
requests in production.

Sampling is set to 0 by default, so tracing stays fully off unless
explicitly enabled.

src/backend/meet/settings.py

@@ -469,6 +469,9 @@ class Base(Configuration):
 
     # Sentry
     SENTRY_DSN = values.Value(None, environ_name="SENTRY_DSN")
+    SENTRY_TRACES_SAMPLE_RATE = values.FloatValue(
+        0.0, environ_name="SENTRY_TRACES_SAMPLE_RATE", environ_prefix=None
+    )
 
     # Easy thumbnails
     THUMBNAIL_EXTENSION = "webp"
@@ -1211,7 +1214,14 @@ class Base(Configuration):
                 dsn=cls.SENTRY_DSN,
                 environment=cls.__name__.lower(),  # build, test, development, production
                 release=get_release(),
-                integrations=[DjangoIntegration()],
+                traces_sample_rate=cls.SENTRY_TRACES_SAMPLE_RATE,
+                integrations=[
+                    DjangoIntegration(
+                        transaction_style="url",
+                        middleware_spans=True,
+                        cache_spans=True,
+                    )
+                ],
             )
             sentry_sdk.set_tag("application", "backend")