Update installing_rabbitmq_centos.md

Updated the EPEL URL repo
Update installing_rabbitmq_centos.md

Updating URL for latest RPM version of RabbitMQ
Update adding_a_metric.md

Fix plugin name
Merge pull request #33 from cdgraff/master

Updated DOC
Merge pull request #34 from cdgraff/patch-1

Update adding_a_metric.md