mark anomalous metrics as such

I added a `bool` field to the `data.Datapoint` struct that will say "yes, I am an anomaly".

This helps applications parse the output channel without creating a second channel that could potentially block if not properly/diligently read from. It also avoids a less-than-ideal `strings.HasSuffix("anomalous")` check.

I'm open to discussing other methods, but this seemed the easiest.