Support peakSamples in query frontend response

[yeya24]

Is your feature request related to a problem? Please describe.
peakSamples field was supported in Prometheus query response long time ago https://github.com/prometheus/prometheus/pull/10369/files#diff-9e1a49d3ecc79eae504d539895c7621d3f45eef192ddd6192d8b304ffa591864R111.

However, it is not supported yet at Cortex. https://github.com/cortexproject/cortex/blob/master/pkg/querier/tripperware/query.proto#L41

We only have totalQueryableSamples and totalQueryableSamplesPerStep.

Describe the solution you'd like
Modify the query frontend protobuf and add the field. Also implement the merge logic accordingly.

[SungJin1212]

@yeya24
Could you assign to me?

[yeya24]

Here you go.

[SungJin1212]

@yeya24
I think the peakSamples should be the largest one among the query responses in the merging process. Because the peakSample is the maximum samples kept in memory during the query process.. What do you think of it?

[yeya24]

Sounds reasonable to aggregate using max not sum.

[SungJin1212]

@yeya24
I have made the pr! (#6188)