route: tweak comment on metric types

Change-Id: I9fb463a5b26e79073a3efc06eea13fd8fcdbf021
Reviewed-on: https://go-review.googlesource.com/50190
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
5 files changed