I recently had an issue in OEM 12c while creating a Metric Extension that was returning more than 5000 records. In OEM 12c versions 126.96.36.199 and 188.8.131.52 there a flood control mechanism in order to limit the maximum number of rows for a metric.
You can also verify this in the agent_inst/sysman/log/gcagent.log file:
2018-10-08 13:38:50,330 [66:F119EDFE:GC.Executor.1 (host:hostname:ll_host_config) (host:hostname:ll_host_config:ECM_OS_COMPONENT)] WARN - Result set exceeding min flood control level
The solution is provided in the MOS Doc Id 1499381.1
We didn’t want to proceed to apply the patch but rather we limited the number of rows from the Metric Extension.
This OMS patch is available also for 13.1 version and the bug states that is fixed in 13.2 versions.