Support query maxSamples limit

[yeya24]

The original promql engine supports specifying maxSamples for each query https://github.com/prometheus/prometheus/blob/main/promql/engine.go#L248 and this is useful to protect the query engine from OOM kill.

It would be nice to support this feature here.

[fatsheep9146]

I'd love to help with this issue.

My idea is that since each operator take a VectorPool to store the temporary results. I think we can cumulate the sample counter when operator store the result into the vector pool.

How do you think about this? @yeya24 @fpetkovski