StatPanels: Add new calculation option for percentage difference (#26369)

* Update fieldReducer.ts

addition of percentage difference to the singlestat panel

* Update time_series2.ts

* Update module.ts

* Update calculations-list.md

* Update docs/sources/panels/calculations-list.md

Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

* Update public/app/plugins/panel/singlestat/module.ts

Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

* Update module.ts

* Update calculations-list.md

* Update calculations-list.md

* Update module.ts

* Update fieldReducer.ts

* Update fieldReducer.ts

* Update fieldReducer.test.ts

* change name to remove wildcard characters

* Update calculations-list.md

* Update time_series2.ts

Fix spelling

* Update module.ts

* Update fieldReducer.ts

* formatting

* Update fieldReducer.ts

* Update fieldReducer.test.ts

* Update fieldReducer.test.ts

Co-authored-by: Diana Payton <52059945+oddlittlebird@users.noreply.github.com>

public/app/core/time_series2.ts

@@ -232,6 +232,7 @@ export default class TimeSeries {
     this.stats.first = null;
     this.stats.delta = 0;
     this.stats.diff = null;
+    this.stats.diffperc = 0;
     this.stats.range = null;
     this.stats.timeStep = Number.MAX_VALUE;
     this.allIsNull = true;
@@ -336,6 +337,7 @@ export default class TimeSeries {
     }
     if (this.stats.current !== null && this.stats.first !== null) {
       this.stats.diff = this.stats.current - this.stats.first;
+      this.stats.diffperc = this.stats.diff / this.stats.first;
     }
 
     this.stats.count = result.length;

public/app/plugins/panel/singlestat/module.ts

@@ -68,6 +68,7 @@ class SingleStatCtrl extends MetricsPanelCtrl {
     { value: 'first', text: 'First' },
     { value: 'delta', text: 'Delta' },
     { value: 'diff', text: 'Difference' },
+    { value: 'diffperc', text: 'Difference percent' },
     { value: 'range', text: 'Range' },
     { value: 'last_time', text: 'Time of last point' },
   ];