[Feature Flags] Support multi-client configs for browser SDKs

[aliu39]

Description

https://docs.sentry.io/platforms/javascript/best-practices/shared-environments/
The browser may have >1 Sentry client per SDK instance. The most common case seems to be browser extensions. We could support this case by isolating flags from different clients, maintaining a WeakMap of <Client, flag evaluations>.

[mydea]

Just some thoughts, generally in such scenarios we do not support global things very well overall. There are a lot of integrations etc. that do not work in such global settings (meaning, they only work for the "main" global app, but not e.g. the browser extension), and it can be a reasonable solution to say that a certain feature is not fully supported in e.g. a browser extension. Not saying this is what we should do, just wanted to point out that sometimes it is simply not really doable well, sadly, so we don't always have a choice :(

[aliu39]

Thanks for the thoughts @mydea , I think the last time I talked about this with @cmanallen , he saw it as an edge case we won't prioritize until popularly requested. I made this ticket as a reminder, but we might end up just not doing it

[aliu39]

Due the reasons above, I'll close this as won't do in the near future. The effort to change our implementation is pretty high for an edge case