Added search metrics from /stats API

Added:
"riak_kv_vnodes_running"
"riak_kv_vnodeq_min"
"riak_kv_vnodeq_median"
"riak_kv_vnodeq_mean"
"riak_kv_vnodeq_max"
"riak_kv_vnodeq_total"

and notes based on http://docs.basho.com/riak/latest/references/appendices/Inspecting-a-Node/