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Doc: Info about $__rate_interval for naive users (#36576)

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* Doc: Info about $__rate_interval for naive users

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**What this PR does / why we need it**:

For Prometheus newbies (i.e. most of us), it seems like $__rate_interval should be widely used, but the documentation doesn't tell us that. So I added more about why it's good, and an explicit recommendation that it can be a good default value for use if (like most of us) you don't really understand how to choose an interval. (Let me know if that's not actually the case, but that's what I took away from https://github.com/grafana/grafana/issues/21417)

Also fixed a few typos in the existing doc.

**Which issue(s) this PR fixes**:

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Fixes #

**Special notes for your reviewer**:

* Edits based on suggestions from @beorn7

* Update docs/sources/datasources/prometheus.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

Co-authored-by: Ivana Huckova <30407135+ivanahuckova@users.noreply.github.com>
Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>
Co-authored-by: Ivana <ivana.huckova@gmail.com>
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docs/sources/datasources/prometheus.md
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@ -127,9 +127,7 @@ For details of what _metric names_, _label names_ and _label values_ are please
> Support for `$__range`, `$__range_s` and `$__range_ms` only available from Grafana v5.3
You can use some global built-in variables in query variables; `$__interval`, `$__interval_ms`, `$__range`, `$__range_s` and `$__range_ms`, see [Global built-in variables]({{< relref "../variables/variable-types/global-variables.md" >}}) for more information. These can be convenient to use in conjunction with the `query_result` function when you need to filter variable queries since
`label_values` function doesn't support queries.
You can use some global built-in variables in query variables, for example, `$__interval`, `$__interval_ms`, `$__range`, `$__range_s` and `$__range_ms`. See [Global built-in variables]({{< relref "../variables/variable-types/global-variables.md" >}}) for more information. They are convenient to use in conjunction with the `query_result` function when you need to filter variable queries since the `label_values` function doesn't support queries.
Make sure to set the variable's `refresh` trigger to be `On Time Range Change` to get the correct instances when changing the time range on the dashboard.
@ -149,11 +147,18 @@ Query: query_result(max_over_time(<metric>[${__range_s}s]) != <state>)
Regex:
```
### Using `$__rate_interval` variable
### Using `$__rate_interval`
> **Note:** Available in Grafana 7.2 and above
The `$__rate_interval` variable is meant to be used in the rate function. It is defined as max( `$__interval` + _Scrape interval_, 4 \* _Scrape interval_), where _Scrape interval_ is the Min step setting (AKA query*interval, a setting per PromQL query), if any is set, and otherwise the \_Scrape interval* as set in the Prometheus data source (but ignoring any Min interval setting in the panel, because the latter is modified by the resolution setting).
`$__rate_interval` is the recommended interval to use in the `rate` and `increase` functions. It will "just work" in most cases, avoiding most of the pitfalls that can occur when using a fixed interval or `$__interval`.
```
OK: rate(http_requests_total[5m])
Better: rate(http_requests_total[$__rate_interval])
```
Details: `$__rate_interval` is defined as max(`$__interval` + _Scrape interval_, 4 \* _Scrape interval_), where _Scrape interval_ is the Min step setting (AKA query_interval, a setting per PromQL query) if any is set. Otherwise, the Scrape interval setting in the Prometheus data source is used. (The Min interval setting in the panel is modified by the resolution setting and therefore doesn't have any effect on _Scrape interval_.) [This article](https://grafana.com/blog/2020/09/28/new-in-grafana-7.2-__rate_interval-for-prometheus-rate-queries-that-just-work/) contains additional details.
### Using variables in queries