[cache metadata]: No indication of what percentage of metadata is in RAM

For the new Cache Metadata feature we don't appear to give the user any indication of how much metadata has been flushed out to disk.

See attached screenshot - while we do show the absolute amount of RAM used for metadata, there doesn't seem to be any indication of how much of the total is still in RAM.

Note: I had a brief look at the available stats (https://github.com/membase/ep-engine/blob/master/docs/stats.org) and couldn't see a stat about total metadata size (flushed to disk); so this may also need ep-engine if there isn't an underlying stat for this.