[kineto] deprecate metdata args from ClientTraceActivity (#55988)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55988
Pull Request resolved: https://github.com/pytorch/kineto/pull/165
as part of the ClientTraceActivity -> GenericTraceActivity migration, move all the metadata fields to JSON encoded string
Test Plan:
- `buck build`
- tested with subsequent diffs
Reviewed By: gdankel
Differential Revision: D27340314
fbshipit-source-id: f55b77a779e4bda1fb8667cb4e0f4252b93af5ea
