API: Adding the histogram implementation to storage_proxy
API: Adding the histogram implementation to storage_proxy

This adds the implementation to the histogram for the storage proxy.
After this patch the following url will be available:
/storage_proxy/metrics/read/latency/histogram
/storage_proxy/metrics/range/latency/histogram
/storage_proxy/metrics/write/latency/histogram

Signed-off-by: Amnon Heiman <ed5fc74c4692051ad0b19749fd7cea6ec2d74b9e@cloudius-systems.com>

rluta/scylla,asias/scylla,guiquanz/scylla,avikivity/scylla,wildinto/scylla,avikivity/scylla,gwicke/scylla,stamhe/scylla,glommer/scylla,linearregression/scylla,guiquanz/scylla,respu/scylla,raphaelsc/scylla,bowlofstew/scylla,dwdm/scylla,kjniemi/scylla,linearregression/scylla,acbellini/scylla,respu/scylla,wildinto/scylla,asias/scylla,justintung/scylla,shaunstanislaus/scylla,acbellini/scylla,senseb/scylla,kjniemi/scylla,kangkot/scylla,rentongzhang/scylla,tempbottle/scylla,glommer/scylla,raphaelsc/scylla,dwdm/scylla,asias/scylla,bowlofstew/scylla,dwdm/scylla,glommer/scylla,guiquanz/scylla,eklitzke/scylla,senseb/scylla,shaunstanislaus/scylla,aruanruan/scylla,victorbriz/scylla,gwicke/scylla,stamhe/scylla,phonkee/scylla,rentongzhang/scylla,victorbriz/scylla,avikivity/scylla,aruanruan/scylla,eklitzke/scylla,duarten/scylla,kjniemi/scylla,capturePointer/scylla,scylladb/scylla,duarten/scylla,gwicke/scylla,tempbottle/scylla,linearregression/scylla,phonkee/scylla,acbellini/scylla,rluta/scylla,shaunstanislaus/scylla,phonkee/scylla,victorbriz/scylla,eklitzke/scylla,scylladb/scylla,stamhe/scylla,senseb/scylla,duarten/scylla,wildinto/scylla,aruanruan/scylla,rluta/scylla,tempbottle/scylla,scylladb/scylla,scylladb/scylla,kangkot/scylla,capturePointer/scylla,capturePointer/scylla,respu/scylla,justintung/scylla,bowlofstew/scylla,kangkot/scylla,raphaelsc/scylla,justintung/scylla,rentongzhang/scylla