* 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>
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