Missing @since tag

Change-Id: I6195dce9cea9a4fed4a48449e655271db68d95c2
Consistency around SdrMetricItem

...where some instances of SDRATTR_TEXT_MINFRAMEHEIGHT etc. where SdrMetricItem
while others where of trivial derived classes SdrTextMinFrameHeightItem etc.,
that appear to only existed for better-named constructors.

Change-Id: If0af79fbb803275ed09fc7b950868c5de7d61615