opentracing: use a consistent name for background processes (#10135)

... otherwise we tend to get a namespace clash between the bg process and the
functions that it calls.

synapse/logging/opentracing.py

@@ -337,6 +337,7 @@ def ensure_active_span(message, ret=None):
 @contextlib.contextmanager
 def noop_context_manager(*args, **kwargs):
     """Does exactly what it says on the tin"""
+    # TODO: replace with contextlib.nullcontext once we drop support for Python 3.6
     yield