[Sessions] Add meta.size to bfetched responses

[lizozom]

Summary

Since we want to avoid stringifying responses to calculate their size, after consulting with @ppisljar, I added a new meta section to the response coming from any bfetch function.
It will contain the response's size in bytes, so it can then be used anywhere it's needed.
Specifically, I intend to use it in #92439 for its cache eviction logic.

I split this out to a separate PR to simplify testing and reviewing.

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• Any UI touched in this PR is usable by keyboard only (learn more about keyboard accessibility)
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• This was checked for cross-browser compatibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)

[ppisljar]

i don't really like it that we are adding things to actual response, i would preffer to se meta information separate from actual responses.

also, i think this information could be useful on the server as well, and i would really prefer to keep our server/client API as similar as possible.

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 86ae706
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Metrics [docs]

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
bfetch	11.1KB	11.2KB	+78.0B

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @lizozom

[streamich]

I'm worried that the responses now may contain this meta property which is not expected. And it is unfortunate to add the BatchResultBase interface, as Peter also mentioned.

Could you please make it such that the result data is not modified, i.e. with the extra meta property? One way that comes to mind could be to return the whole response, like so (instead of just response.result):

That way you can use response.json.length to get the size of the raw response. And response.result to get the response data as before.

[lizozom]

@streamich yep, thanks!
I agree with you both and that's going to be my current approach 🙏🏻

[lizozom]

@elasticmachine merge upstream

[kibanamachine]

ignoring request to update branch, pull request is closed