API: Add read, write, range estimated histogram implementation for storage_proxy
API: Add read, write, range estimated histogram implementation for storage_proxy

This patch adds the implmentation for the read, write and range
estimated histogram and total latency.

After this patch the following url will be available:
/storage_proxy/metrics/read/estimated_histogram/
/storage_proxy/metrics/read
/storage_proxy/metrics/write/estimated_histogram/
/storage_proxy/metrics/write
/storage_proxy/metrics/range/estimated_histogram/
/storage_proxy/metrics/range

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

kjniemi/scylla,raphaelsc/scylla,scylladb/scylla,kjniemi/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,raphaelsc/scylla,scylladb/scylla,kjniemi/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla