Event tags flex report

Event-tags report.

Runs a dashboard-style aggregation and returns the same visualization payload the web dashboard
uses for a widget preview. Provide one dimension, one or more same-family metrics, a
timeRange ({ startTime, endTime } in milliseconds since epoch) and optional locations/cameras
scope. metrics[].targetIds come from the dictionary endpoints (/v1/dictionary/objects,
/v1/dictionary/alerts, /v1/dictionary/eventTags). The response visualizationData.data is an
array of { xLabel, xGrid, yGrid }
grouped by the dimension.