insights: introduced query cost and priority to distribute resources across multiple insights

Created by: coury-clark

Closes #21483 (closed)

Introduces two concepts to code insights: priority and cost. priority is a simple mechanism to distribute query processing evenly across insights. cost is largely for analysis right now, but in the future (assuming we can come up with useful heuristic) we will use it to determine if insights can be calculated immediately or need background processing.