[AE-1184] Add contextual targeting metrics

[abedef]

This PR adds two metrics, taxonomy and categories, to support contextual targeting for direct sold stories (ticket).

Measurement Description	Data Collection Category	Tracking Bug #
ad.taxonomy is a taxonomy identifier for the contextual categories (e.g. "IAB-3.0"). May be null if no contextual information was provided.	Category 1	https://bugzilla.mozilla.org/show_bug.cgi?id=2017204
ad.categories is a comma-separated list of contextual category identifiers from the taxonomy (e.g. "210,315"). May be null if no contextual information was provided.	Category 2	https://bugzilla.mozilla.org/show_bug.cgi?id=2017204

• Data review pending approval
• Sensitive Data Review complete

[mashalifshin]

Total nitpick on the PR title, but could you please reword to something like "Add contextual targeting metrics"? (this PR adds metrics to existing pings, but title kinda sounds like you're adding new pings)

[mashalifshin]

@abedef I met with Nick Kypreos today to talk through the "live test" plan and he was strongly in favor of including the taxonomy column in the first pass for future-proofing version changes, even if it would just store the same value. So I think since Glenda confirmed there's no additional cost here, let's leave it.

[mashalifshin]

This PR has gone through the data steward review process, and the sensitive data review process, and received approval, so now I can go ahead and Approve this PR.

See links in the PR description for data review details.