As per the documentation/annotation found in the XML Schema for plugin descriptors (rhq-plugin.xsd):
default collection intervals must be 30s or higher. See line 201-214 of rhq-plugin.xsd where it defines the defaultInterval schema:
So, had you been using an XML Editor that validates your plugin descriptor against this XSD schema, it should have flagged that as an invalid value.
We shouldn't silently ignore this though - if we aren't logging an error message at plugin deployment time, we should be. It would be a problem if we do not log an error to let you know why it failed.
I just tried deploying a plugin with one of the metric's default collection interval set to 20000. The following warning was printed in the Server log:
08:58:22,067 WARN [AgentPluginScanner] Failed to scan agent plugin [/home/ips/Projects/rhq/dev-container/jbossas/server/default/deploy/rhq.ear/rhq-downloads/rhq-plugins/jopr-jboss-as-plugin-4.1.0-SNAPSHOT.jar] found on filesystem. Skipping. Cause: org.rhq.core.clientapi.agent.PluginContainerException: Could not successfully parse the plugin descriptor [META-INF/rhq-plugin.xml found in plugin jar at [file:/home/ips/Projects/rhq/dev-container/jbossas/server/default/deploy/rhq.ear/rhq-downloads/rhq-plugins/jopr-jboss-as-plugin-4.1.0-SNAPSHOT.jar]
It correctly states that it could not parse the plugin descriptor, but it doesn't give any details as to what was wrong with the plugin descriptor. We can probably print something more informative here. I'll take a peek at the code.