fetch list of nodes only once while adding metric metadata

it eliminates >150 http requests per existing bucket

Change-Id: Ic3aabe87272c9511f514354563ba3b588d44a427
Reviewed-on: http://review.couchbase.org/24523
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>