runtime/metrics: change unit on *-by-size metrics to match bucket unit

This change modifies the *-by-size metrics' units to be based off the
bucket's unit (bytes) as opposed to the unit of the counts (objects).
This convention is more in-line with distributions in other metrics
systems.

Change-Id: Id3b68a09f52f0e1ff9f4346f613ae1cbd9f52f73
Reviewed-on: https://go-review.googlesource.com/c/go/+/282352


Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Knyszek <mknyszek@google.com>