Update readdirplus metric to use histogram instead of counter

[dannycjones]

Today, the readdirplus.entries metric will simple count all the entries returned in the interval (roughly 5 seconds). However, it would make more sense to return some statistics on how many entries were returned in a readdirplus request when there was more than one in a given interval.

The metric is emitted here:

mountpoint-s3/mountpoint-s3/src/fuse.rs

Line 228 in ef20898

metrics::counter!("fuse.readdirplus.entries").increment(count as u64);

We should update this code to emit the metric using a histogram instead.