Introducing a builder API for scopes which encourages better use by g…

…uiding users to add metadata to scopes only at the point of creation. This also deprecates several existing methods and strongly discourages post-creation modification (there are still reasons people might want this though, especially for things like enabling more logging, which could happen asynchronously).

Tidied up some inconsistent documentation and documentation errors, and reordered some methods to just bring things together a bit better.

I think Gerrit uses the existing API (if not, they should migrate from the ad hoc API they had), so while this CL shouldn't break anyone, it's worth calling out for Gerrit.

RELNOTES=Improving the scoped logging context API and deprecating some methods.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=325061198