Stats to show how much data is written to disk by KV-Engine

We have no true stat that tells us how much data Couchbase is writing to disk.

There is ep_io_write_bytes:

ep_io_write_bytes Number of bytes written (key + values)

However I do not feel that is useful and even misleading. A write to couchstore includes a ton more data, such as the B-tree update and metadata of the file, also the data is compressed to using snappy.

(It would be nice if we also track the writes that other Couchbase processes do, like view updates and compaction).