Code review comment for ~superalpaca/ols-charm-deps:SN1460-add-prometheus-multiproc-dir-config

Thanks for the reviews everyone!

I think it makes sense to update the config option to be a boolean (false/off by default), so I've gone ahead and done that. If this option is set, then gunicorn will override the PROMETHEUS_MULTIPROC_DIR env vars, provided we have more than 1 worker.

I'm not too sure how this will interact with talisker, but it's probably best to not set this when talisker is being used. Maybe adding a check for talisker (like https://pastebin.canonical.com/p/yVRpQ5k9jt/) could work?