Reduce impact of ExplainCostsRouter and many concurrent queries #10
Description
Currently 100,000 queries in trino-lb running through ExplainCostsRouter cause 100,000 queries on the underlying Trino cluster used for the query estimations, which can lead to DOS.
To prevent this we need to let users specify the cluster group used for the explain queries instead of a single cluster. This should also take care of queuing explain queries.
An alternative solution could be to enter trino-lb itself as the explain target cluster, but users would need to be careful to not creating routing loops 🤷♂️ To prevent this a dedicated trino-lb only for explain queries could be set up. Seems unnecessary complicated though.