Initialize finished counter at zero

[bilbof]

Sets initial count of task finished state to zero. This enables acquiring the rate from zero to one (particularly useful if you want to alert on task failures).

We're using the Prometheus statsd-exporter. Since counters are usually used with a PromQL function like rate, it's important
that counters are initialized at zero, otherwise when a task finishes the rate function will not have a previous value to compare the state count to.

For example, what we'd like to do, which tells us the failure rate of tasks over time:

sum by (dag_id, task_id) (rate(airflow_ti_finish{state='failed'}[1h])) > 0

Two useful posts on this subject:
https://www.robustperception.io/why-predeclare-metrics
https://www.section.io/blog/beware-prometheus-counters-that-do-not-begin-at-zero/

[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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[ashb]

This increments that counter by 0. That doesn't seem right https://statsd.readthedocs.io/en/v3.3/reference.html#StatsClient.incr

[bilbof]

Incrementing the timer by zero tells the receiver that the metric exists (sets it at zero) which is useful in cases where you want to capture the rate change from 0 to 1. Initializing metrics at zero is best practice for Prometheus metrics (which is how we're consuming the Statsd metrics from Airflow jobs).

I've tested that should work with the Statsd Prometheus exporter locally:

# first shell
docker run -p 9102:9102 -p 9125:9125  -it prom/statsd-exporter
# second shell
echo "foo:0|c" | nc localhost 9125
curl localhost:9102/metrics | grep foo

# Result:
# TYPE foo counter
> foo 0

[ashb]

Oh, somehow I thought this was changing the existing metric we emitted. Looking at it again I see it's not. Clearly not.

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[bilbof]

Thanks @ashb, I rebased to the latest main.

[bilbof]

@ashb just a gentle nudge about this one - tests are passing now 😃

[boring-cyborg]

Awesome work, congrats on your first merged pull request!

[jedcunningham]

Thanks @bilbof, congrats on your first commit 👍