feat(blackhole sink): accept metric events, too

[thoughtpolice]

The Vector website indicates that the blackhole sink accepts both log
and metric events, but this isn't true: it only accepts logs. There's no
reason this should be the case.

In particular, for metric sources (such as statsd in my case) that
want to accept/transform metrics but just send everything to /dev/null
for the moment (e.g. when debugging your configuration), there's no easy
way to do this, resulting in an annoying error in the startup topology
check.

I also like enabling blackholes with high print_amount settings
anyway, so that my journald logs show some progress occasionally. This
will allow me to send all streams to one blackhole that prints that
info.

This patch simply takes events and distinguish metrics, adding them to
the total event count, and taking the total length of the .to_string()
method for the byte size, AKA the json representation. I believe this is
roughly the right metric, since it is basically a measure of "how many
bytes this stream will roughly push out", which will generally be some
rendered form of the metric data -- such as JSON.

[binarylogic]

Thanks @thoughtpolice! We'll get this reviewed.

@loony-bean do you mind reviewing this? Thanks.

[lukesteensen]

Thanks for this! We still have some work to do with the Any type for sources and transforms, but we should definitely be using it here.

[lukesteensen]

@thoughtpolice Would you mind running cargo fmt and pushing the changes? Would be happy to merge once that check is green.