Paid Media schema updates for PMax, dimensional breakdowns, and impression share docs

[ejsuncy]

Summary

Schema updates to address five open questions from the Paid Media XDM Schema review (internal wiki):

Issue 1: PMax Asset Performance (HIGH)

• Add performance_max_asset_group to xdm:adGroupType enum to represent Google Performance Max Asset Groups (the Ad Group equivalent in PMax)
• dynamic_assembly already exists in xdm:experienceType for PMax dynamic ad composition
• PMax asset-level quantitative metrics are available via the asset_group_asset resource (impressions, clicks, CTR, conversions, conversion value, cost — segmentable by ad_network_type). Note: ad_group_ad_asset_view does not work for PMax; asset_group_asset is purpose-built for it.
• Critical requirement: Asset Group must be ingested and stored as a proper entity (Ad Group lookup with adGroupType: PERFORMANCE_MAX_ASSET_GROUP and a valid adGroupID). Without this, asset_group_asset metric rows cannot be joined to asset/campaign metadata.

Issue 2: Keywords / Extensions / Negatives (LOW)

• No schema changes — use additionalDetails with documented naming conventions
• Future core-paid-media-search-details field group deferred to v2

Issue 3: Summary Metrics & Breakdowns (HIGH)

• Add xdm:adNetworkType (search, display, youtube, shopping, etc.) to core-paid-media-dimensional-breakdowns
• Add xdm:adSlot (top, absolute_top, other) to core-paid-media-dimensional-breakdowns
• Both are soft enums, null for platforms that do not support them (e.g., Meta)

Issue 4: Impression Share Metrics (LOW)

• Add field-level descriptions to impression share metrics in core-paid-media-cost-metrics noting availability at Campaign and Ad Group levels only

Issue 5: Google Video Asset Handling (MEDIUM-HIGH)

• No schema changes needed — existing fields already support this
• YouTube URL + thumbnail as immediate approach; video binary download investigation deferred to later iteration