add CheckpointOverheadRatio to kvstore

Change-Id: I41d720e8cc23c17f7133add1c7b4d0e3c718c40b
Reviewed-on: https://review.couchbase.org/c/perfrunner/+/170975
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Sean Corrigan <sean.corrigan@couchbase.com>

Author: bochun

cbagent/collectors/kvstore_stats.py

@@ -99,7 +99,8 @@ class KVStoreStats(Collector):
         "NReadIOsSetPerSec",
         "ActiveIndexBlocksSize",
         "NBlocksCached",
-        "CheckpointOverheadPeriod"
+        "CheckpointOverheadPeriod",
+        "CheckpointOverheadRatio"
     )
     METRICS_AVERAGE_PER_NODE_PER_SHARD = (
         "ReadAmp",