[investigate] output/drop: make `drop reason` more informative
With drops being caused by elements such as the exception policies, `drop.reason` needs more data
to be actually informative.
Two examples of drop reasons that seem to not show enough to be conclusive to users:
drop.reason: "applayer error"
drop.reason: "stream memcap"
I've created this with the [investigate] "tag" because it actually seems that each of those reasons could only happen under specific cases
- and yet, when debugging, the info might seem incomplete...