Metric for write key conflicts

[ammuv]

From: Brainstorm discussion

Use an histogram whose buckets b{le x} represent count of conflict le x. Use k2::ExponentialHistogram for this purpose.

Update the histogram by tracking competing WIs.

Each WI that won the push inherits conflict count from the losing WI and increments it by 1.

The histogram is updated every time a finalization occurs.

Note: this metric allows us to track high conflicts per key but does not help us identify which key caused this.