fix fstream metrics reporting.
fix fstream metrics reporting.

Message-Id: <ef3630bdfd63165df7fb0d2d13a5d3272a26757d@scylladb.com>

scylladb/seastar,dreamsxin/seastar,avikivity/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar,cloudius-systems/seastar,dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar