Docs: Added $__rate_interval to Prometheus help (#28030)

* Docs: Added $__rate_interval to Prometheus help

The variable '$__rate_interval' was added in Grafana 7.2.0. This commit
adds the missing help information to Grafana's Prometheus PromQueryEditor.

Signed-off-by: David Calvert <davidcalvertfr@gmail.com>

* Docs: Fixed line length in PromQueryEditor.tsx

This commit will fix line length of my previous commit to make CI green
again.

Signed-off-by: David Calvert <davidcalvertfr@gmail.com>

* Docs: Fixed ci error

Fixed CI error due to my previous commits

Signed-off-by: David Calvert <davidcalvertfr@gmail.com>

docs/sources/features/datasources/prometheus.md

@@ -46,7 +46,7 @@ Open a graph in edit mode by clicking the title > Edit (or by pressing `e` key w
 | ------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | _Query expression_  | Prometheus query expression, check out the [Prometheus documentation](http://prometheus.io/docs/querying/basics/).                                                                                                                                                                                                                                                                                                                                                                                        |
 | _Legend format_     | Controls the name of the time series, using name or pattern. For example `{{hostname}}` is replaced with the label value for the label `hostname`.                                                                                                                                                                                                                                                                                                                                                        |
-| _Min step_          | An additional lower limit for the [`step` parameter of Prometheus range queries](https://prometheus.io/docs/prometheus/latest/querying/api/#range-queries) and for the `$__interval` variable. The limit is absolute and not modified by the _Resolution_ setting.                                                                                                                                                                                                                                        |
+| _Min step_          | An additional lower limit for the [`step` parameter of Prometheus range queries](https://prometheus.io/docs/prometheus/latest/querying/api/#range-queries) and for the `$__interval` and `$__rate_interval` variables. The limit is absolute and not modified by the _Resolution_ setting.                                                                                                                                                                                                                                        |
 | _Resolution_        | `1/1` sets both the `$__interval` variable and the [`step` parameter of Prometheus range queries](https://prometheus.io/docs/prometheus/latest/querying/api/#range-queries) such that each pixel corresponds to one data point. For better performance, lower resolutions can be picked. `1/2` only retrieves a data point for every other pixel, and `1/10` retrieves one data point per 10 pixels. Note that both _Min time interval_ and _Min step_ limit the final value of `$__interval` and `step`. |
 | _Metric lookup_     | Search for metric names in this input field.                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
 | _Format as_         | Switch between `Table`, `Time series`, or `Heatmap`. `Table` will only work in the Table panel. `Heatmap` is suitable for displaying metrics of the Histogram type on a Heatmap panel. Under the hood, it converts cumulative histograms to regular ones and sorts series by the bucket bound.                                                                                                                                                                                                             |

public/app/plugins/datasource/prometheus/components/PromQueryEditor.tsx

@@ -136,7 +136,8 @@ export class PromQueryEditor extends PureComponent<Props, State> {
               tooltip={
                 <>
                   An additional lower limit for the step parameter of the Prometheus query and for the{' '}
-                  <code>$__interval</code> variable. The limit is absolute and not modified by the "Resolution" setting.
+                  <code>$__interval</code> and <code>$__rate_interval</code> variables. The limit is absolute and not
+                  modified by the "Resolution" setting.
                 </>
               }
             >

public/app/plugins/datasource/prometheus/components/__snapshots__/PromQueryEditor.test.tsx.snap

@@ -52,7 +52,11 @@ exports[`Render PromQueryEditor with basic options should render 1`] = `
             <code>
               $__interval
             </code>
-             variable. The limit is absolute and not modified by the "Resolution" setting.
+             and 
+            <code>
+              $__rate_interval
+            </code>
+             variables. The limit is absolute and not modified by the "Resolution" setting.
           </React.Fragment>
         }
         width={7}