This makes it possible to instantiate object metrics using JOG. This does NOT check values against the provided schema. Thus while this enables object metrics, they behave slightly different than the otherwise builtin ones: `set` and `get` works as expected from JavaScript. However on `set` nothing is checked. You can set arbitrary values and they will be recorded and returned. Any error checking will fail. That's why we still special-case those tests in `test_Glean.js`. Differential Revision: https://phabricator.services.mozilla.com/D237552