database: fix collectd metrics for clustering key filter
database: fix collectd metrics for clustering key filter

Same instance name was used for exported metrics, which is
definitely wrong. Checked it works properly now via collectd
exporter.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>
Message-Id: <471a36706113af60aeba86fb56a365feb4dab31a.1477086706.git.1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

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