Log stack traces when a missing opentracing span is detected

Make it easier to track down where opentracing spans are going missing
by including stack traces in the logs.

synapse/logging/opentracing.py

@@ -339,6 +339,7 @@ def ensure_active_span_inner_2(*args, **kwargs):
                     "There was no active span when trying to %s."
                     " Did you forget to start one or did a context slip?",
                     message,
+                    stack_info=True,
                 )
 
                 return ret