This patch adds two new monkit metric:
* piece_writer_io: the sum of the time, which is spent with io.Write during a piece upload (excluding the fs sync of the commit)
* piece_writer_hash: the sum of the time, which is spent with hashing
The second is especially important. My storagenode (hosted on a cloud server) spend ~30 ms on hasing data, piece_write_io time is usually 5ms for me.
These metrics can help us to identify the reason of slownes on storagenode sides.
Both of these depend on the size of the piece. To make it more meaningfull without exploding the cardinality, I created a few size categories and classified the pieces based on these. Measurements shows that it can provide usefull results (>2MB uploads are usually 23-28 ms).
Change-Id: Ifa1c205a490046655bcc34891003e7b43ed9c0bc
