https://abseil.io/tips/153
https://quuxplusone.github.io/blog/2020/12/21/using-directive/#using-directives
https://google.github.io/styleguide/cppguide.html#Namespaces

As it is right now, this directive make it possible for streams:: names to hide mongo:: names.

Or for mongo:: names to clash with global namespace names!

That means that maintainers of mongo:: headers will break your code in the future by defining names that conflict with names in the global namespace.