correction in above first url:
http://<ambari-server>:8080/api/v1/clusters/<cluster-name>?fields=metrics/cpu[1430844925,1430848525,15]&_=1430848532904
correction in second url
http://<ambari-servert>:8080/api/v1/clusters/<cluster-name>?fields=metrics/network[1430844925,1430848525,15]&_=1430848532904