Add most missing type hints to synapse.util (#11328)

synapse/util/metrics.py

@@ -64,6 +64,13 @@ in_flight = InFlightGauge(
     sub_metrics=["real_time_max", "real_time_sum"],
 )
 
+
+# This is dynamically created in InFlightGauge.__init__.
+class _InFlightMetric(Protocol):
+    real_time_max: float
+    real_time_sum: float
+
+
 T = TypeVar("T", bound=Callable[..., Any])
 
 
@@ -180,7 +187,7 @@ class Measure:
         """
         return self._logging_context.get_resource_usage()
 
-    def _update_in_flight(self, metrics) -> None:
+    def _update_in_flight(self, metrics: _InFlightMetric) -> None:
         """Gets called when processing in flight metrics"""
         assert self.start is not None
         duration = self.clock.time() - self.start