Fix typing for notifier (#8064)

synapse/util/metrics.py

@@ -15,6 +15,7 @@
 
 import logging
 from functools import wraps
+from typing import Any, Callable, Optional, TypeVar, cast
 
 from prometheus_client import Counter
 
@@ -57,8 +58,10 @@ in_flight = InFlightGauge(
     sub_metrics=["real_time_max", "real_time_sum"],
 )
 
+T = TypeVar("T", bound=Callable[..., Any])
 
-def measure_func(name=None):
+
+def measure_func(name: Optional[str] = None) -> Callable[[T], T]:
     """
     Used to decorate an async function with a `Measure` context manager.
 
@@ -76,7 +79,7 @@ def measure_func(name=None):
 
     """
 
-    def wrapper(func):
+    def wrapper(func: T) -> T:
         block_name = func.__name__ if name is None else name
 
         @wraps(func)
@@ -85,7 +88,7 @@ def measure_func(name=None):
                 r = await func(self, *args, **kwargs)
             return r
 
-        return measured_func
+        return cast(T, measured_func)
 
     return wrapper