Disclose render mode

[benmccann]

Describe the problem

The performance characteristics of SPA vs SSR apps is very different. While many frameworks only support one or the other, SvelteKit can run in different modes and supports both. It quite hard to compare SvelteKit to an SSR-only framework or SPA-only framework because our numbers are grouped together despite these being totally different deployment modes. Being able to report on this would help us generate better reports at https://cwvtech.report/ to understand where users are succeeding at creating fast sites and where we can better guide them.

HTTPArchive/cwv-tech-report#32 (comment)

Describe the proposed solution

Mark feature usage for stuff like ssr|prerendered|spa|mpa and enhanced-img:
https://www.w3.org/TR/user-timing/#dfn-mark_feature_usage

This could be controlled by an option similar to Svelte's discloseVersion option.

Alternatives considered

No response

Importance

nice to have

Additional Information

No response

[jasongitmail]

Possibly related: It'd be useful to see a tree of routes with the rendering method listed for each, to review for performance opportunities, likely as part of build output.