[v10.2.x] Prometheus: Fix $__rate_interval calculation

[tolzhabayev]

Manual Backport b607a4e from #77234

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Background Information

$__rate_interval calculation is explained here in detail.
Shortly it is max($__interval + Scrape interval, 4 * Scrape interval) where Scrape interval is the “Min step” setting (also known as query*interval, a setting per PromQL query) if any is set. Otherwise, Grafana uses the Prometheus data source’s “Scrape interval” setting.
Why we need that in the first place is explained here in this beautiful blog post.
$__interval calculation is explained here.

For all data sources, we have a Min Interval value. The default value of this is 15s or you can set it differently in data source settings. To be able to interpolate $__interval we also send intervalMs. This is always the millisecond value of Interval in the panel below.

Last 2 Days time range

In the Prometheus query editor, we also have Min Step for queries.

Users might want to override Min Interval or Min Step.

Min Interval setting has precedence over the scrape interval as set in the data source.
Min Step setting has precedence over the Min Interval.

Backend receives that as interval. This can be seen in Query Inspector.

So let's use the values below as example.

Data source Scrape Interval	30s
Time Range	Last 2 Days
Interval (calculated Automatically)	1m
Min Interval (Overriden)	100s
Min Step (Overriden)	150s

Based on the documentation let's put the values in the equation and find the $__rate_interval value.

max($__interval + query.interval, 4 * query.interval)
max(       60   +  150 ,          4 * 150)          

So this should give the value as 600s. And the step value we send to Prometheus should be 150s => 2m30s. Let's see it in the query inspector:

As you can see something is not quite right. So this PR is fixing it. Let's switch to this branch and run our query again.

Who is this feature for?

Prometheus users

Special notes for your reviewer:

• I updated the variable names since they were quite confusing
• I put some comments

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.