[Operations][CI] Should report page load asset size

[thomasneirynck]

Describe the feature:

CI should report the page load asset size. Current bundle metrics indicate how a plugin contributes to overal code size, but not when this code get loaded.

Practically speaking, it is relevant that plugins do not cause async code loading for code that is not required on pages that do not need it.

Consider using the kibana start page as the reference page.

Describe a specific use case for the feature:

Ensure developers take care of how and when code chunks are loaded when structuring code.

Seems a similar effort from a few years ago tried to achieve this #66224

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)