Group a watch by entity

Add groupBy: [service] under a watch's spec and retain service in its source fields. Input such as {"service":"checkout","latency_ms":350} then evaluates a separate state for that service. IDs retain scalar type and field presence.

Each entity keeps its own continuity, incident, baseline, and timers. A watch can contain an open incident for one service and a normal condition for another. Inspect the entity summaries and their pagination instead of assigning one entity's status to the whole watch.

Set limits.maxEntities for expected cardinality. Selected fields and group values are retained evidence: avoid unnecessary identifiers. Quota pressure rejects work or produces explicit unknown state; it does not pretend a partial exact window is complete. See limits and retention.