Support subqueries on Vertical Sharding

[alanprot]

Is your proposal related to a problem?

As for right now any query that has a subquery are not sharded.

For instance the following query is shardable avg(kube_pod_container_status_running) by (pod, namespace, cluster_id) but not avg(kube_pod_container_status_running[1h:1m]) by (pod, namespace, cluster_id)

See:

thanos/pkg/querysharding/analyzer.go

Lines 92 to 94 in 25d91c1

	case *parser.SubqueryExpr:
	isShardable = false
	return fmt.Errorf("expressions with subqueries are not shardable")

Describe the solution you'd like

Seems like queries like avg(kube_pod_container_status_running[1h:1m]) by (pod, namespace, cluster_id) could also take advantage of the query vertical sharding.