Replace underscore with lodash - custom metrics

ee/app/assets/javascripts/custom_metrics/components/custom_metrics_form_fields.vue

 <script>
 import { GlFormInput, GlLink, GlFormGroup, GlFormRadioGroup, GlLoadingIcon } from '@gitlab/ui';
-import { debounce } from 'underscore';
+import { debounce } from 'lodash';
 import { __, s__ } from '~/locale';
 import Icon from '~/vue_shared/components/icon.vue';
 import csrf from '~/lib/utils/csrf';