nbacquey

> @nbacquey Have you noticed that doctests are failing? Thanks for noticing ; doctests should be fine now

> I would love to read a bit more on practical uses of this feature! Do you use this in tracing scenarios? Observability and Telemetry? Indeed, my first motivation for...

I don't remember my specific usecase, but I hadn't thought about using `tweakResponse` like that, thanks for the tip!

> Do I understand correctly that this would require changing a type in servant and that the new combinator you describe would need to be based off a patched servant?...

I rebased on the new version of #1556, which reinstates "real" type hints, instead of `CaptureSingle`/`CaptureList`

Added a fix to prevent `WithRoutingHeader` from breaking `IsElem`

@tchoutri @alpmestan Could I have your position about this PR?

> @nbacquey Hi! After a cursory review, I'd like to ask for a changelog entry, as it is my understanding that you add a new public module `Servant.Server.TypeErrors` that you...

This later version should work, the test suite passes. The `Fragment a` keyword was also recently introduced in Servant, and hadn't a `HasMock` instance yet, which I've added.