In EM 11 (and earlier releases) we could use User Defined Metrics to extend the “out-of-box” monitoring capabilities. One of the drawbacks of User Defined Metrics in pre 12c releases is that they only applied to the Host and Database (Instance and Cluster Database) Target types.
EM 12c introduces Metric Extensions for the same goal, however supports any Target type.
Metric Extension Library
In EM 11 (and earlier releases) there isn’t a central location to store and maintain User Defined Metrics. A User Defined Metric could only be created as a Target related object, meaning you had to pick a certain Target (Host or Database) and create your UDM. Then by including your UDM in a Monitoring Template you would be able to deploy the UDM to other Targets.
By identifying one specific Host and one specific Database we would simulate a Library for our UDM’s. Normally this would be the OMS (Management Service) Server (or 1 of them when using multiple OMS’s) for Host UDM’’s and the OMR (Management Repository) Database Target for Database related UDM’s.
In EM 12c, a Metric Extension Library has been introduced for this purpose!