[Feature] Allow order control in createAggConfig (#3160)

* [Feature] Allow order control in `createAggConfig`

Add new optional boolean param that adds the new `AggConfig` to the beginning of the array rather than the end.
Makes it easier to work with Pie or other visualizations with `Schemas` that set `mustBeFirst`

Signed-off-by: Josh Romero <rmerqg@amazon.com>

* [Chore] Add changelog

Signed-off-by: Josh Romero <rmerqg@amazon.com>

Signed-off-by: Josh Romero <rmerqg@amazon.com>
Co-authored-by: Qingyang(Abby) Hu <abigailhu2000@gmail.com>
(cherry picked from commit b750846)
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

# Conflicts:
#	CHANGELOG.md

src/plugins/data/common/search/aggs/agg_configs.test.ts

@@ -117,7 +117,7 @@ describe('AggConfigs', () => {
       expect(ac.aggs).toHaveLength(3);
     });
 
-    it('adds new AggConfig entries to AggConfigs by default', () => {
+    it('adds new AggConfig entries to end of AggConfigs by default', () => {
       const configStates = [
         {
           enabled: true,
@@ -136,6 +136,7 @@ describe('AggConfigs', () => {
         schema: 'split',
       });
       expect(ac.aggs).toHaveLength(2);
+      expect(ac.aggs[1].schema).toBe('split');
     });
 
     it('does not add an agg to AggConfigs if addToAggConfigs: false', () => {
@@ -161,6 +162,55 @@ describe('AggConfigs', () => {
       );
       expect(ac.aggs).toHaveLength(1);
     });
+
+    it('adds new AggConfig entries to beginning of AggConfigs if mustBeFirst: true', () => {
+      const configStates = [
+        {
+          enabled: true,
+          type: 'histogram',
+          params: {},
+        },
+      ];
+
+      const ac = new AggConfigs(indexPattern, configStates, { typesRegistry });
+      expect(ac.aggs).toHaveLength(1);
+
+      ac.createAggConfig(
+        {
+          enabled: true,
+          type: 'terms',
+          params: {},
+          schema: 'split',
+        },
+        { mustBeFirst: true }
+      );
+      expect(ac.aggs).toHaveLength(2);
+      expect(ac.aggs[0].schema).toBe('split');
+    });
+
+    it('does not add an agg to AggConfigs if addToAggConfigs: false and mustBeFirst: true', () => {
+      const configStates = [
+        {
+          enabled: true,
+          type: 'histogram',
+          params: {},
+        },
+      ];
+
+      const ac = new AggConfigs(indexPattern, configStates, { typesRegistry });
+      expect(ac.aggs).toHaveLength(1);
+
+      ac.createAggConfig(
+        {
+          enabled: true,
+          type: 'terms',
+          params: {},
+          schema: 'split',
+        },
+        { addToAggConfigs: false, mustBeFirst: true }
+      );
+      expect(ac.aggs).toHaveLength(1);
+    });
   });
 
   describe('#getRequestAggs', () => {

src/plugins/data/common/search/aggs/agg_configs.ts

@@ -130,10 +130,10 @@ export class AggConfigs {
 
   createAggConfig = <T extends AggConfig = AggConfig>(
     params: CreateAggConfigParams,
-    { addToAggConfigs = true } = {}
+    { addToAggConfigs = true, mustBeFirst = false } = {}
   ) => {
     const { type } = params;
-    let aggConfig;
+    let aggConfig: AggConfig;
 
     if (params instanceof AggConfig) {
       aggConfig = params;
@@ -145,8 +145,12 @@ export class AggConfigs {
       });
     }
 
+    const addAggConfig = () => {
+      return mustBeFirst ? this.aggs.unshift(aggConfig) : this.aggs.push(aggConfig);
+    };
+
     if (addToAggConfigs) {
-      this.aggs.push(aggConfig);
+      addAggConfig();
     }
 
     return aggConfig as T;