Add deprecated emit_metric() helper to cog interface

[aron]

This helps with the transition between the main cog branch and the experimental async branch. Models built for the async branch can be run on either without code changes.

New models should still be using the internal current_scope().record_metric() interface.

[philandstuff]

This looks good. However it will conflict with #2066 because I've updated tests that work with async models to specify a minimum python version (see for ex https://github.com/replicate/cog/pull/2066/files#diff-922f17529df722a8c4e93788b47410def10f269ce3cd5cff2f768feb2f066336R80). I think the easiest thing would be to change the base of this PR to support-concurrent-predictions-in-child, then get the tests passing (probably just by specifying minimum python version). Then we can either merge this into the support-concurrent-predictions-in-child branch before merging #2066, or merge #2066 first and then this.

[aron]

Perfect, I'll cherry-pick and fix the tests in #2066

[aron]

@philandstuff I'm going to merge this into main, then clean-up #2026 ready to be merged.