feat: Use Datadog's tag feature to aggregate metrics better

[Zylphrex]

Using the tags feature allows there to be less unique metric names and simplifies the process of creating a dashboard on Datadog since new graphs can be automatically created based on the tags.

[Zylphrex]

@mikeclarke I'm not too clear on what Datadog let's you do with tags, but from what I can see, this will let you just select the dag.duration metric then you can simply say one graph per dag and they'll all be created for you.

[mikeclarke]

this looks right to me, maybe we can tag the operator class as well?