Prometheus: add $__rate_interval variable (#26937)

* Add rate interval variable to prometheus data source

* Add tests + auto complete

* Fix prometheus tests

* Add doc

* Modify test title

* Modify kbn method name after merge

docs/sources/features/datasources/prometheus.md

@@ -110,6 +110,9 @@ Populate a variable with the instances having a certain state over the time rang
 Query: query_result(max_over_time(<metric>[${__range_s}s]) != <state>)
 Regex:
 ```
+### Using `$__rate_interval` variable
+
+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).
 
 ### Using variables in queries
 

public/app/plugins/datasource/prometheus/datasource.test.ts

@@ -1601,9 +1601,28 @@ describe('PrometheusDatasource', () => {
           text: expectedRangeSecond * 1000,
           value: expectedRangeSecond * 1000,
         },
+        __rate_interval: {
+          text: '75s',
+          value: '75s',
+        },
       });
     });
   });
+
+  describe('The __rate_interval variable', () => {
+    it('should be 4 times the scrape interval if interval + scrape interval is lower', () => {
+      const { __rate_interval } = ds.getRateIntervalScopedVariable(23, 23);
+      expect(__rate_interval.value).toBe('60s');
+    });
+    it('should be interval + scrape interval if 4 times the scrape interval is lower', () => {
+      const { __rate_interval } = ds.getRateIntervalScopedVariable(56, 56);
+      expect(__rate_interval.value).toBe('71s');
+    });
+    it('should fall back to 60s if interval is 0', () => {
+      const { __rate_interval } = ds.getRateIntervalScopedVariable(0, 0);
+      expect(__rate_interval.value).toBe('60s');
+    });
+  });
 });
 
 describe('PrometheusDatasource for POST', () => {

public/app/plugins/datasource/prometheus/datasource.ts

@@ -335,13 +335,18 @@ export class PrometheusDatasource extends DataSourceApi<PromQuery, PromOptions>
     const intervalFactor = target.intervalFactor || 1;
     // Adjust the interval to take into account any specified minimum and interval factor plus Prometheus limits
     const adjustedInterval = this.adjustInterval(interval, minInterval, range, intervalFactor);
-    let scopedVars = { ...options.scopedVars, ...this.getRangeScopedVars(options.range) };
+    let scopedVars = {
+      ...options.scopedVars,
+      ...this.getRangeScopedVars(options.range),
+      ...this.getRateIntervalScopedVariable(interval, minInterval),
+    };
     // If the interval was adjusted, make a shallow copy of scopedVars with updated interval vars
     if (interval !== adjustedInterval) {
       interval = adjustedInterval;
       scopedVars = Object.assign({}, options.scopedVars, {
         __interval: { text: interval + 's', value: interval + 's' },
         __interval_ms: { text: interval * 1000, value: interval * 1000 },
+        ...this.getRateIntervalScopedVariable(interval, minInterval),
         ...this.getRangeScopedVars(options.range),
       });
     }
@@ -380,6 +385,16 @@ export class PrometheusDatasource extends DataSourceApi<PromQuery, PromOptions>
     return query;
   }
 
+  getRateIntervalScopedVariable(interval: number, minInterval: number) {
+    let intervalInSeconds = minInterval === interval ? kbn.intervalToSeconds(this.interval) : minInterval;
+    // if intervalInSeconds === 0 then we should fall back to the default 15 seconds
+    if (intervalInSeconds === 0) {
+      intervalInSeconds = 15;
+    }
+    const rateInterval = Math.max(interval + intervalInSeconds, 4 * intervalInSeconds);
+    return { __rate_interval: { text: rateInterval + 's', value: rateInterval + 's' } };
+  }
+
   adjustInterval(interval: number, minInterval: number, range: number, intervalFactor: number) {
     // Prometheus will drop queries that might return more than 11000 data points.
     // Calculate a safe interval as an additional minimum to take into account.

public/app/plugins/datasource/prometheus/language_provider.test.ts

@@ -124,6 +124,7 @@ describe('Language completion provider', () => {
         {
           items: [
             { label: '$__interval', sortText: '$__interval' }, // TODO: figure out why this row and sortText is needed
+            { label: '$__rate_interval', sortText: '$__rate_interval' },
             { label: '1m', sortText: '00:01:00' },
             { label: '5m', sortText: '00:05:00' },
             { label: '10m', sortText: '00:10:00' },

public/app/plugins/datasource/prometheus/promql.ts

@@ -2,6 +2,7 @@ import { CompletionItem } from '@grafana/ui';
 
 export const RATE_RANGES: CompletionItem[] = [
   { label: '$__interval', sortText: '$__interval' },
+  { label: '$__rate_interval', sortText: '$__rate_interval' },
   { label: '1m', sortText: '00:01:00' },
   { label: '5m', sortText: '00:05:00' },
   { label: '10m', sortText: '00:10:00' },