[ResponseOps] optimize building ecsFieldMap (#159251)

resolves https://github.com/elastic/kibana/issues/157772

Previously on my M1 Macbook, the `ecsFieldMap` constant takes about 170
ms to be created. It appears this is another case of `array::reduce()`
and object spread causing performance issues.

Converting to (essentially) a filter and map got the time down to 4 ms.

Slightly critical even though this is only ever run once - because it's
run at startup time, and blocking everything else from running.

See:

-
https://www.richsnapp.com/article/2019/06-09-reduce-spread-anti-pattern
-
https://prateeksurana.me/blog/why-using-object-spread-with-reduce-bad-idea/

packages/kbn-alerts-as-data-utils/src/field_maps/ecs_field_map.ts

@@ -11,25 +11,23 @@ import { EcsMetadata, FieldMap } from './types';
 
 const EXCLUDED_TYPES = ['constant_keyword'];
 
-export const ecsFieldMap: FieldMap = Object.keys(EcsFlat).reduce((acc, currKey) => {
-  const value: EcsMetadata = EcsFlat[currKey as keyof typeof EcsFlat];
-
-  // Exclude excluded types
-  if (EXCLUDED_TYPES.includes(value.type)) {
-    return acc;
-  }
-
-  return {
-    ...acc,
-    [currKey]: {
-      type: value.type,
-      array: value.normalize.includes('array'),
-      required: !!value.required,
-      ...(value.scaling_factor ? { scaling_factor: value.scaling_factor } : {}),
-      ...(value.ignore_above ? { ignore_above: value.ignore_above } : {}),
-      ...(value.multi_fields ? { multi_fields: value.multi_fields } : {}),
-    },
-  };
-}, {});
+export const ecsFieldMap: FieldMap = Object.fromEntries(
+  Object.entries(EcsFlat)
+    .filter(([_, value]) => !EXCLUDED_TYPES.includes(value.type))
+    .map(([key, _]) => {
+      const value: EcsMetadata = EcsFlat[key as keyof typeof EcsFlat];
+      return [
+        key,
+        {
+          type: value.type,
+          array: value.normalize.includes('array'),
+          required: !!value.required,
+          ...(value.scaling_factor ? { scaling_factor: value.scaling_factor } : {}),
+          ...(value.ignore_above ? { ignore_above: value.ignore_above } : {}),
+          ...(value.multi_fields ? { multi_fields: value.multi_fields } : {}),
+        },
+      ];
+    })
+);
 
 export type EcsFieldMap = typeof ecsFieldMap;