Slight tweaks to the domain analytics value

[mjriley]

Technical Summary

This is a slight tweak on the previous solution created in #35748. I was bothered by the duplication, but couldn't find any better way to handle it. After looking through some documentation on custom tags, however, I realized as is a cleaner version that eliminates the duplication.

Feature Flag

Safety Assurance

Safety story

Verified locally that the domain value was being populated correctly on both the raw_doc page (which needs domain) and the app builder panel (which needs request.domain).

Automated test coverage

No tests

QA Plan

No QA

Rollback instructions

• This PR can be reverted after deploy with no further considerations

Labels & Review

• Risk label is set correctly
• The set of people pinged as reviewers is appropriate for the level of risk of the change

[jingcheng16]

Nice. I learned firstof tag too.

[jingcheng16]

@mjriley Do you know why the test failed on this PR not on the two previous PRs that touches metrics?

[mjriley]

@mjriley Do you know why the test failed on this PR not on the two previous PRs that touches metrics?

Yep. The change here introduced a new context variable, metrics_domain (because it felt risky to override domain with request.domain). We had a few tests that verified the entire context for some app builder pages, and so they were failing when detecting the new variable