Improve Description of dag_processing.processes Metric in Docs

[dvfeinblum-betterment]

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Hey folks; this is a very very small proposal regarding the description of the dag_processing.processes metric emitted by airflow. It is a delta value, not the absolute value, and that can lead to very difficult interpretation of what this metric actually means.

If you look at the data this metric emits, it's sometimes negative (which doesn't seem to make a huge amount of sense since it's the "count" of dag processing processes spun off by processor.py). I don't claim this change to be the best rephrasing, but I thought it'd make sense to have a convo on this pull itself to figure out the best solution.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
Here are some useful points:

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[dvfeinblum-betterment]

Also a note for maintainers, I based my understanding of this metric off a discussion here

apache/skywalking#10341

since, again, the documentation about this metric is incredible sparse from the official docs.

[dvfeinblum-betterment]

Oh also; one question for maintainers. It just dawned on me that this metric (the number of active dag processor processes) is kinda calculable by just looking at airflow.dag_processing.last_duration.count, rendering this metric maybe not super useful? Just sorta thinking out loud. I'll stop bumping this pull now.

[potiuk]

I think @ferruzzi - this might be something that you can drag your attention to while refreshing/updating metrcs.

[ferruzzi]

Very sorry for the delay. Yeah, thanks for updating that. A lot of the metrics are not particularly well documented and I am working on that right now. Your new description looks good to me.

You may also be interested in joining the discussion over here. After I finish adding OpenTelemetry support, we want to make a pass and help clean up things like this and a couple that either don't work at all or don't work as the description might indicate. 👍

[ferruzzi]

I like it.

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.

[dvfeinblum-betterment]

Hey folks; appreciate the quick turn around! I'll be sure to take a peek at the other documentation work going on. I'm a bit of a wordsmith and would be happy to make additiona contributions. Cheers 🎉

[ferruzzi]

We can always use more eyes on the documentation helping clarify stuff. 👍 I might tap you when it comes time to looking at that page.