Bug 1878140 - Make sendBeacon fallback the default uploader.

[perrymcmanis144]

Relies on existing tests.

Prior changes did not include doc changes. @rosahbruno what doc changes would you like to see, if any?

Pull Request checklist

• Quality: Make sure this PR builds and runs cleanly.
  • Inside the glean/ folder, run:
    • npm run test Runs all tests
    • npm run lint Runs all linters
• Tests: This PR includes thorough tests or an explanation of why it does not
• Changelog: This PR includes a changelog entry to CHANGELOG.md or an explanation of why it does not need one
• Documentation: This PR includes documentation changes, an explanation of why it does not need that or a follow-up bug has been filed to do that work
  • Documentation should be added to The Glean Book when making changes to Glean's user facing API
    • When changes to the Glean Book are necessary, link to the corresponding PR on the mozilla/glean repository
  • Documentation should be added to the Glean.js developer documentation when making internal code changes

[rosahbruno]

Relies on existing tests.

We could probably create a test where we set something like global.navigator.sendBeacon = undefined and ensure that the fetch call is made. I don't think this is a make or break, but just something we could do. Similar tests exist in sendbeacon_fallback_uploader.spec.ts.

what doc changes would you like to see, if any?

We are currently making a list of things that we need to add for the new Glean.js docs. I can add this to the list. I don't think we need anything specific for this now. I see in the SDK book there is a section for custom uploaders where we could maybe add a note about this - but I think we can just address it better in the new docs.

[rosahbruno]

LGTM, Nice work on this! 💯