Turn one event into multiple metrics

[lukesteensen]

This is a little funky, but it seemed to hit the right spot of non-invasive, flexible, and no perf impact (I was worried but the benches say I'm ok). Basically, transforms now have the option to implement a transform_into method where they can push as many events as they want into an output vec. The weird part is that this "overrides" the transform function but you still have to implement it.

Reasons I didn't do other things:

• Returning a vec from transform required a bunch of changes and made most transforms more annoying to write, plus required allocating that vec no matter what
• Passing in the output channel would mean transforms have to become more complicated stream adapters since send is async
• Making a whole new stream adapter so we could reuse the same output vec would have been a lot more work and the benchmarks showed that we're not any slower than before
• Doing a properly layered two-trait thing for transform and transform into would have gotten rid of the weirdness, but that also seemed like too much work for the moment. I think it's more likely that we rework this before too long than someone gets confused about the two methods.

Closes #379

[LucioFranco]

check stable got sigkilled :(

[LucioFranco]

@lukesteensen I bumped the box size here #555

[binarylogic]

Nice, I didn't realize this actually fixed the log_to_metric transform 👍