feat: Add is_log and is_metric conditions

[Jeffail]

This adds two new conditions is_log and is_metric, which are fairly self explanatory. They will allow writers of unit tests to check the actual type of an event coming out of certain transforms.

It will also allow users to create swimlanes that separate logs from metrics to different outputs. However, for swimlanes this currently isn't useful at all as it is forced to deal only in logs until #421 is solved.

[binarylogic]

This makes sense! I like it. Would check_fields apply to metrics tags or would we add a new check_tags type?

[Jeffail]

Yeah check_fields tests against tags when it receives metrics.

[binarylogic]

Ah, yeah our terminology isn't consistent here. Ex: add_fields and add_tags transforms. We previously talked about unifiying the term but felt it was misleading since they operate differently. I'm curious what you think.

[Jeffail]

For transforms I'd say it helps to be more explicit. Right now conditions are kind of hidden in that you can live a successful and enriched life without knowing there's anything other than check_fields (and you might not even know it's called that).

As we expand it might be worth eventually making an explicit check_tags condition, not because it's super important to be explicit but more so for discoverability. If we end up with a list of conditions we want people to know they're able to check metric tags by glancing at the types.