Support max samples limit

[yeya24]

#452 supports tracking peak samples during an execution of the query.

What we are still missing from the old Promql engine is max samples limit enforced by flag query.max-samples, which can be used to protect the query engine from getting OOM killed.

https://github.com/prometheus/prometheus/blob/main/cmd/prometheus/main.go#L512

[harry671003]

Is this useful in the engine? Each step is only keeping a batch of 10 samples per series in memory.