make blackhole sink's print_amount field optional

[JeanMertz]

While testing out a local Vector configuration in #230 (comment), I ran into #3885.

That got me thinking, I wonder if we should consider making print_amount an optional field. Its purpose is orthogonal to what the actual sink is supposed to do, and given that this sink seems like a good fit for quickly testing something, the less configuration it requires, the better.

[lukesteensen]

I would really like to get rid of this option entirely. I don't find it useful at all. I think we could have the sink periodically emit how many messages it has received (though even that could be done better with a reduce transform?) or some other form of feedback, but I'm not sure what would be most useful.

[JeanMertz]

I actually agree with you @lukesteensen. One reason I've seen people wanting this is so that their tooling doesn't consider Vector to be stuck if it no longer produces output.

I would say for that we have a much better solution now, the internal_metrics source.

Given that, I think we should just purge this field, and the output this sink generates, entirely.