Secondary Metrics should by default to only look at exposed users

[jurajmajerik]

Feature request

Is your feature request related to a problem?

I know that Neil recently made a change to provide a convenient way to create an exposure cohort that can then be applied to all secondary metrics associated with the experiment. I think this is a good first step however I think we need to go further. I understand that some customers may not want this but for larger customers I think this is the default expectation and I would expect it to be a project wide setting, e.g. experiment secondary metrics are by default limited to users in the exposure cohort.

I think it would be a particularly poor experience if for every experiment that we set up we have to create the exposure cohort and then apply it to secondary metrics. It would also cause a proliferation of cohort creation and more tedious experiment cleanup

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Additional context

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]