We were having contributions of metricsets with the release level in
the description, the cause was that our generator scripts did it this
way.

Add release tag to metricset templates instead of using the description
field for this purpouse.

Make beta the release level for new modules by default. Experimental
should be kept just for real experiments or in some corner cases. The
usual level for a new metricset that collects metrics from a known
existing service or technology should be beta (or ga if it is really
complete).