Fix force_tracing_for_users config when using MAS (#18334)

This is a copy of what we do for internal auth, and we should figure out
a way to deduplicate some of this stuff:


https://github.com/element-hq/synapse/blob/dd05cc55eedbf086ae224a13c9ae9f0332d96b1f/synapse/api/auth/internal.py#L62-L110

Author: erikjohnston

changelog.d/18334.bugfix

@@ -0,0 +1 @@
+Fix `force_tracing_for_users` config when using delegated auth.

synapse/api/auth/msc3861_delegated.py

@@ -45,6 +45,7 @@
 )
 from synapse.http.site import SynapseRequest
 from synapse.logging.context import make_deferred_yieldable
+from synapse.logging.opentracing import active_span, force_tracing, start_active_span
 from synapse.types import Requester, UserID, create_requester
 from synapse.util import json_decoder
 from synapse.util.caches.cached_call import RetryOnExceptionCachedCall
@@ -177,6 +178,7 @@ def __init__(self, hs: "HomeServer"):
         self._http_client = hs.get_proxied_http_client()
         self._hostname = hs.hostname
         self._admin_token: Callable[[], Optional[str]] = self._config.admin_token
+        self._force_tracing_for_users = hs.config.tracing.force_tracing_for_users
 
         # # Token Introspection Cache
         # This remembers what users/devices are represented by which access tokens,
@@ -363,6 +365,55 @@ async def get_user_by_req(
         allow_guest: bool = False,
         allow_expired: bool = False,
         allow_locked: bool = False,
+    ) -> Requester:
+        """Get a registered user's ID.
+
+        Args:
+            request: An HTTP request with an access_token query parameter.
+            allow_guest: If False, will raise an AuthError if the user making the
+                request is a guest.
+            allow_expired: If True, allow the request through even if the account
+                is expired, or session token lifetime has ended. Note that
+                /login will deliver access tokens regardless of expiration.
+
+        Returns:
+            Resolves to the requester
+        Raises:
+            InvalidClientCredentialsError if no user by that token exists or the token
+                is invalid.
+            AuthError if access is denied for the user in the access token
+        """
+        parent_span = active_span()
+        with start_active_span("get_user_by_req"):
+            requester = await self._wrapped_get_user_by_req(
+                request, allow_guest, allow_expired, allow_locked
+            )
+
+            if parent_span:
+                if requester.authenticated_entity in self._force_tracing_for_users:
+                    # request tracing is enabled for this user, so we need to force it
+                    # tracing on for the parent span (which will be the servlet span).
+                    #
+                    # It's too late for the get_user_by_req span to inherit the setting,
+                    # so we also force it on for that.
+                    force_tracing()
+                    force_tracing(parent_span)
+                parent_span.set_tag(
+                    "authenticated_entity", requester.authenticated_entity
+                )
+                parent_span.set_tag("user_id", requester.user.to_string())
+                if requester.device_id is not None:
+                    parent_span.set_tag("device_id", requester.device_id)
+                if requester.app_service is not None:
+                    parent_span.set_tag("appservice_id", requester.app_service.id)
+            return requester
+
+    async def _wrapped_get_user_by_req(
+        self,
+        request: SynapseRequest,
+        allow_guest: bool = False,
+        allow_expired: bool = False,
+        allow_locked: bool = False,
     ) -> Requester:
         access_token = self.get_access_token_from_request(request)