Bug 1688698 - Add 'source' to TopSites schemas in Contextual Services

[ncloudioj]

Checklist for reviewer:

• Commits should reference a bug or github issue, if relevant (if a bug is referenced, the pull request should include the bug number in the title)
• If adding a new field, the field should have a description (see Bug 1652842 - Add a blocklist field so the environment schema works with pre- and post-bug-1647225 data #576 for an example)
• Scan the PR and verify that no changes (particularly to .circleci/config.yml) will cause environment variables (particularly credentials) to be exposed in test logs
• If the PR comes from a fork, trigger the integration CI test by pushing this revision as discussed in the README and review the report posted in the comments.

For glean changes:

• Update templates/include/glean/CHANGELOG.md

This is a follow up for #656

r? @sunahsuh @BenWu

[BenWu]

Looks good to me. Can you also add this to the proposal?

[sunahsuh]

Looks fine to me – only comment is, will we expand the options for this field in the future? If so, how will we remember to keep the list updated here? Just wondering if being more permissive default would make sense, or if filtering out pings with sources outside this list is very important.

[ncloudioj]

Will we expand the options for this field in the future? If so, how will we remember to keep the list updated here? Just wondering if being more permissive default would make sense, or if filtering out pings with sources outside this list is very important.

Good point, though I don't think we will expand this list for the foreseeable future. In case that we do so at some point and we forget to update it here, we should be able to capture it by the validation error monitors (we'd like to have this as one of the system health monitors).

[sunahsuh]

Okay – I think @fbertsch can help point you to doctype-specific validation error monitors