[Fleet] Add support for fields of type aggregate_metric_double (#154920)

## Summary

Add support for fields of type `aggregate_metric_double` in EPM.

Change in package spec introduced in
https://github.com/elastic/package-spec/pull/500.

Fixes https://github.com/elastic/kibana/issues/154867
Closes https://github.com/elastic/package-spec/issues/457

### Checklist

Delete any items that are not applicable to this PR.

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios.

### How to test

* Modify a package to include a field with type
`aggregate_metric_double`, for example like this:
```
    - name: some_metric
      type: aggregate_metric_double
      metrics: [ "min", "max", "sum", "value_count" ]
      default_metric: "max"
```
* Install elastic package with this branch of the package-spec included:
https://github.com/elastic/package-spec/pull/500, for this, from an
elastic-package working directory:
* `go mod edit -replace
github.com/elastic/package-spec/v2=github.com/elastic/package-spec@main`
  * `go mod tidy`
  * `make install`
* Build the package with `elastic-package build -v`.
* Install the package with `elastic-package install -v`.
* Check that the template contains the expected field.

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>

x-pack/plugins/fleet/server/services/epm/elasticsearch/template/template.test.ts

@@ -1034,6 +1034,28 @@ describe('EPM template', () => {
     expect(JSON.stringify(mappings)).toEqual(JSON.stringify(metaFieldMapping));
   });
 
+  it('tests processing field of aggregate_metric_double type', () => {
+    const fieldLiteralYaml = `
+    - name: aggregate_metric
+      type: aggregate_metric_double
+      metrics: ["min", "max", "sum", "value_count"]
+      default_metric: "max"
+    `;
+    const fieldMapping = {
+      properties: {
+        aggregate_metric: {
+          metrics: ['min', 'max', 'sum', 'value_count'],
+          default_metric: 'max',
+          type: 'aggregate_metric_double',
+        },
+      },
+    };
+    const fields: Field[] = safeLoad(fieldLiteralYaml);
+    const processedFields = processFields(fields);
+    const mappings = generateMappings(processedFields);
+    expect(JSON.stringify(mappings)).toEqual(JSON.stringify(fieldMapping));
+  });
+
   it('tests priority and index pattern for data stream without dataset_is_prefix', () => {
     const dataStreamDatasetIsPrefixUnset = {
       type: 'metrics',

x-pack/plugins/fleet/server/services/epm/elasticsearch/template/template.ts

@@ -326,6 +326,14 @@ function _generateMappings(
             const dateMappings = generateDateMapping(field);
             fieldProps = { ...fieldProps, ...dateMappings, type: 'date' };
             break;
+          case 'aggregate_metric_double':
+            fieldProps = {
+              ...fieldProps,
+              metrics: field.metrics,
+              default_metric: field.default_metric,
+              type: 'aggregate_metric_double',
+            };
+            break;
           default:
             fieldProps.type = type;
         }

x-pack/plugins/fleet/server/services/epm/fields/field.ts

@@ -40,6 +40,10 @@ export interface Field {
   dimension?: boolean;
   default_field?: boolean;
 
+  // Fields specific of the aggregate_metric_double type
+  metrics?: string[];
+  default_metric?: string;
+
   // Meta fields
   metric_type?: string;
   unit?: string;