storage_proxy: update correct statistics on range reads
storage_proxy: update correct statistics on range reads

Fixes #2167

Message-Id: <043b8ebe9092c59185ef86b2034353026a95e205@scylladb.com>

scylladb/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla