Include min/max/average file size in Searchable Snapshots Stats API

[tlrx]

This pull request adds the minimum, maximum and average length of files in the Searchable Snapshots Stats API:

 {
      "file_ext" : "fdt",
      "num_files" : 31,
      "total_size" : 417698246269,
      "min_size" : "480mb",
      "min_size_in_bytes" : 503370620,
      "max_size" : "12.9gb",
      "max_size_in_bytes" : 13947816041,
      "avg_size" : "12.5gb",
      "avg_size_in_bytes" : 13474136976,
      ...
}

These information are useful in understanding why some data is still being read from the blob store when the cached blob in .snapshot-blob-cache should have been sufficient to server all the read operations.

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[ywelsch]

Very useful to add this info to the stats API. I've left some minor comments w.r.t. consistency between total and min/max fields. I'm also wondering we should fold CacheIndexInputStats.{total_size,min_size,max_size,avg_size} into a size section (i.e. have fields "total,min,max,avg" under a "size" object)

[tlrx]

@ywelsch thanks for your feedback. I've updated the code, can you please have another look when you have time?

[ywelsch]

LGTM

[tlrx]

Thanks Yannick