New APIs to add/remove metric readers at run-time (#4863)

* New APIs to add/remove metric readers at run-time

* Updated typehints for `_collect` callback.
Added Changelog entry for the public API

* Updated tests and fixed lint

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Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>
Co-authored-by: Emídio Neto <9735060+emdneto@users.noreply.github.com>
Co-authored-by: Aaron Abbott <aaronabbott@google.com>