improvement(stress_exporter): rename keyspace to _keyspace
improvement(stress_exporter): rename keyspace to _keyspace

Prometheus sort labels names. Due to that, label name "keyspace"
is not displayed on popup on panel. Rename label name to _keyspace

scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests