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
.map(ProfileService::docIdToUid)
                                    .sorted()
                                    .collect(Collectors.joining(",")),
                                subject.getUser().principal(),
                                subject.getRealm().getName(),
                                subject.getRealm().getDomain() == null
                                    ? ""
                                    : (" under domain [" + subject.getRealm().getDomain().name() + "]")
                            )
                        )
                    );
                }
            }
        }
        return new SubjectSearchResultsAndErrors<>(versionedDocs, errors);
    }
    private SearchRequest buildSearchRequestForSubject(Subject subject) {
        final BoolQueryBuilder boolQuery = QueryBuilders.boolQuery()
            .filter(QueryBuilders.termQuery("user_profile.user.username.keyword", subject.getUser().principal()));
        if (subject.getRealm().getDomain() == null) {
            boolQuery.filter(QueryBuilders.termQuery("user_profile.user.realm.type", subject.getRealm().getType()));
            if (false == isFileOrNativeRealm(subject.getRealm().getType())) {
                boolQuery.filter(QueryBuilders.termQuery("user_profile.user.realm.name", subject.getRealm().getName()));
            }
        } else {
            logger.debug(
                () -> format(
                    "searching existing profile document for user [%s] from any of the realms [%s] under domain [%s]",
                    subject.getUser().principal(),
                    collectionToCommaDelimitedString(subject.getRealm().getDomain().realms()),