Observation (#1998)

* Initial posted draft of observation proposal

* Fixup a few typos and expand the macro section to a bit more detail and incorperate some of the feedback from the initial pitch phase

* spelling fix

Co-authored-by: Remy Demarest <[email protected]>

* Additional prose around general introduction, dependencies (and possible options of the proposal) tracking, and ABI/API impact

* spelling fix

Co-authored-by: Remy Demarest <[email protected]>

* Add type annotations and a note about same

* Some grammar fixes

* Add detail about ObservedChanges and ObservedValues

* Revisions, remove dependencies(for:) default impl

* More revisions re Ben's feedback

* Update proposals/NNNN-observability.md

* Update and rename NNNN-observability.md to 0396-observability.md

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Co-authored-by: Remy Demarest <[email protected]>
Co-authored-by: Nate Cook <[email protected]>
Co-authored-by: Ben Cohen <[email protected]>

Author: 4 people