The issue is version incompatibility between OnCommand API servers 1.0 and OnCommand Performance Manager 2.0GA. Officially, API 1.0 only supports OPM up to 1.1 per the Interoperability Matrix, which is why you don't get metrics.
If you get the API 1.1 Release Candidate you will get metrics. API 1.0 did work with the OPM 2.0 Release Candidate just fine, which was really a problem when they switched to GA and stuff didn't work quite right.
This particular issue killed me during evaluation of the tools - the API 1.1 RC didn't come out until almost two months after OPM 2.0 and OCUM 6.3 GA versions, as I recall. The release candidates of both OCUM and OPM worked fine with API 1.0, then update to the GA versions, stopped getting metrics to a whole bunch of custom collection scripts in my environment.
Also be aware that OC-API 1.1 RC1 is not "perfect". Example - the REST APIs still all use API version 1.0 in the URI. The front page which documents the APIs hasn't changed. BUT the APIs have actually changed, especially regarding the metrics. There are tons MORE metrics available that are NOT documented in the front end web page. The new metrics are listed in the documentation. There are also a couple of subtle naming changes to the old metrics. You will have to examine your code to see if changes are needed due to the additions and changes. You cannot depend on versioning of the REST API because they didn't change the version in the REST call.
I've opened cases about all of these items - I'm hoping that the GA version addresses these topics. For the moment, just to get data I'm having to rework significant chunks of code that were written depending on a stable and/or versioned API set typical of good REST implementations.
I hope this helps you.
Lead Storage Engineer
Huron Legal | Huron Consulting Group
NCDA, NCIE - SAN Clustered, Data Protection
Kudos and accepted solutions are always appreciated.