Make read-repair stats be 'proplist' instead of 'value'

'value' is actually something that should practically never be
used in exometer function metrics. In this case, it causes the
return value to be wrong if the exometer:aggregate() function doesn't
find any detailed stats to aggregate.
Merge pull request #1067 from basho/uw-fix-read-repair-stats

Make read-repair stats be 'proplist' instead of 'value'

Reviewed-by: jburwell