MB-39505: Update string_view key to StatDef in StatCollector
MB-39505: Update string_view key to StatDef in StatCollector

The other addStat methods were moved to using StatDefs in an earlier
patch, one was missed.

Change-Id: I74ee1fbea092a7a739d06be485834b5d3fbf2358
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/135516
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine