Type hints for the remaining two files in synapse.http. (#11164)

* Teach MyPy that the sentinel context is False

This means that if `ctx: LoggingContextOrSentinel`
then `bool(ctx)` narrows us to `ctx:LoggingContext`, which is a really
neat find!

* Annotate RequestMetrics

- Raise errors for sentry if we use the sentinel context
- Ensure we don't raise an error and carry on, but not recording stats
- Include stack trace in the error case to lower Sean's blood pressure

* Make mypy pass for synapse.http.request_metrics

* Make synapse.http.connectproxyclient pass mypy

Co-authored-by: reivilibre <oliverw@matrix.org>

synapse/logging/context.py

@@ -220,7 +220,7 @@ class _Sentinel:
         self.scope = None
         self.tag = None
 
-    def __str__(self):
+    def __str__(self) -> str:
         return "sentinel"
 
     def copy_to(self, record):
@@ -241,7 +241,7 @@ class _Sentinel:
     def record_event_fetch(self, event_count):
         pass
 
-    def __bool__(self):
+    def __bool__(self) -> Literal[False]:
         return False