This commit adds a few new metrics that should be helpful in diagnosing
OOMs such as seen in cockroachdb#80155, and, down the road, for experimenting with
(and ultimately implementing) solutions to cockroachdb#79215.

`cr.store.raft.rcvd.queued_bytes`: gauge (sum of size of all entries waiting to be handed to raft)
`cr.store.raft.rcvd.stepped_bytes`: counter (sum of size of all entries handed to RawNode.Step)
`cr.store.raft.rcvd.dropped_bytes`: counter (sum of size of all entries that were dropped because recv queue filled up)

Release note: None