As we are rolling out the user profile feature, we'd like to expand the [Usage API](https://www.elastic.co/guide/en/elasticsearch/reference/current/usage-api.html) to report usage stats of user profiles such as (1) number of enabled user profiles and (2) number of user profiles that are [activated](https://www.elastic.co/guide/en/elasticsearch/reference/current/security-api-activate-user-profile.html) over the past X days. 

Response of the Usage API already contains a `security` field that groups relevant stats from security related features. We expect the new stats for user profiles should be added under it as well, for example:
```
{
  "security: {
    "user_profile": {
      "activated": 78,
      "recently_activated": 42  <1>
    }
  }
}
```
<1> The use of `recently_` is chosen to avoid the field to be tied to a specific time period so it is easier to evolve the definition in future.

x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/profile/ProfileService.java
final SearchRequest searchRequest = buildSearchRequestForSuggest(request, parentTaskId);
            frozenProfileIndex.checkIndexVersionThenExecute(
                listener::onFailure,
                () -> executeAsyncWithOrigin(
                    client,
                    getActionOrigin(),
                    SearchAction.INSTANCE,
                    searchRequest,
                    ActionListener.wrap(searchResponse -> {
                        final SearchHits searchHits = searchResponse.getHits();
                        final SearchHit[] hits = searchHits.getHits();
                        final SuggestProfilesResponse.ProfileHit[] profileHits;
                        if (hits.length == 0) {
                            profileHits = new SuggestProfilesResponse.ProfileHit[0];
                        } else {
                            profileHits = new SuggestProfilesResponse.ProfileHit[hits.length];
                            for (int i = 0; i < hits.length; i++) {
                                final SearchHit hit = hits[i];
                                final VersionedDocument versionedDocument = new VersionedDocument(
                                    buildProfileDocument(hit.getSourceRef()),
                                    hit.getPrimaryTerm(),
                                    hit.getSeqNo()
                                );
                                profileHits[i] = new SuggestProfilesResponse.ProfileHit(
                                    versionedDocument.toProfile(request.getDataKeys()),
                                    hit.getScore()
                                );
                            }
                        }
                        listener.onResponse(