fix(starlette): Use url transaction source for host routing (#7266)

### Description

When host routing, we don't have a route and instead fall back to the
request path for the transaction name. This should be given transaction
source `url` (for raw URLs) instead of the current value of `route` (for
parameterized routes).

This matters for #7183, where the mis-categorized transaction source
would lead to bad data in `http.route`. (Bug found by Cursor in
#7183 (comment)
when reviewing that PR).

Author: mjq

sentry_sdk/integrations/starlette.py

@@ -843,21 +843,24 @@ async def json(self: "StarletteRequestExtractor") -> "Optional[Dict[str, Any]]":
             return None
 
 
-def _transaction_name_from_router(scope: "StarletteScope") -> "Optional[str]":
+def _transaction_name_and_source_from_router(
+    scope: "StarletteScope",
+) -> "Tuple[Optional[str], TransactionSource]":
     router = scope.get("router")
     if not router:
-        return None
+        return None, TransactionSource.ROUTE
 
     for route in router.routes:
         match = route.matches(scope)
         if match[0] == Match.FULL:
             try:
-                return route.path
+                return route.path, TransactionSource.ROUTE
             except AttributeError:
-                # routes added via app.host() won't have a path attribute
-                return scope.get("path")
+                # Host routes have no path template, so fall back to the
+                # concrete request path and classify it as a URL.
+                return scope.get("path"), TransactionSource.URL
 
-    return None
+    return None, TransactionSource.ROUTE
 
 
 def _set_transaction_name_and_source(
@@ -872,7 +875,7 @@ def _set_transaction_name_and_source(
             name = transaction_from_function(endpoint) or None
 
     elif transaction_style == "url":
-        name = _transaction_name_from_router(request.scope)
+        name, source = _transaction_name_and_source_from_router(request.scope)
 
     if name is None:
         name = _DEFAULT_TRANSACTION_NAME
@@ -891,7 +894,6 @@ def _get_transaction_from_middleware(
         name = transaction_from_function(app.__class__)
         source = TransactionSource.COMPONENT
     elif integration.transaction_style == "url":
-        name = _transaction_name_from_router(asgi_scope)
-        source = TransactionSource.ROUTE
+        name, source = _transaction_name_and_source_from_router(asgi_scope)
 
     return name, source

tests/integrations/starlette/test_starlette.py

@@ -1051,6 +1051,31 @@ def test_transaction_style(
     assert event["transaction_info"] == {"source": expected_source}
 
 
+def test_host_route_path_has_url_source(sentry_init, capture_events):
+    sentry_init(
+        integrations=[StarletteIntegration(transaction_style="url")],
+        traces_sample_rate=1.0,
+    )
+
+    async def hosted_endpoint(request):
+        return starlette.responses.JSONResponse({"status": "ok"})
+
+    subapp = starlette.applications.Starlette(
+        routes=[starlette.routing.Route("/users/{user_id}", hosted_endpoint)]
+    )
+    app = starlette.applications.Starlette(
+        routes=[starlette.routing.Host("subapp", subapp)]
+    )
+
+    events = capture_events()
+    client = TestClient(app)
+    client.get("/users/123456", headers={"Host": "subapp"})
+
+    (event,) = events
+    assert event["transaction"].endswith("/users/123456")
+    assert event["transaction_info"] == {"source": "url"}
+
+
 @pytest.mark.parametrize(
     "test_url,expected_error,expected_message",
     [