Druid doesn’t emit any metric for rows scanned per query

[ravisharm]

Often times question comes up on quantifying the cost of a Druid query. Other products in similar domain e.g. AWS Redshift Serverless, Amazon Managed Service for Prometheus, Timestream etc. emit a metric that gives data scanned per query as a way to represent cost of the query. Druid needs to emit a similar metric. This issue proposes that metric to be number of rows scanned per query. Query profile mechanism also advocated Druid emitting metric on rows scanned. This metric can greatly help in determining costly queries and help understand how customers use Druid.