[FTS] stats: Add descriptive stats on key, value, and batch size for Moss Store

Moss Store is tuned to work well with Bleve and FTS. Current perf numbers look promising for the case where batches are mostly full.

There's some question what batches look like under real world scenarios with FTS.

1. Theoretically, if a user creates a new full text index, the batches will be large for quite some time until the index is built. It would be good to get some confirmation of this.
2. Once an index is built, and if the rate of mutations is small, let's say 3 ops/sec, there may be a lot of small batches. That should be investigated to rule out inefficient use of system resources under the current aggressive compaction strategy.