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 SearchHit hit = hits[0];
                    final ProfileDocument profileDocument = buildProfileDocument(hit.getSourceRef());
                    if (subject.canAccessResourcesOf(profileDocument.user().toSubject())) {
                        versionedDocs.add(
                            new Tuple<>(subject, new VersionedDocument(profileDocument, hit.getPrimaryTerm(), hit.getSeqNo()))
                        );
                    } else {
                        assert false : "this should not happen";
                        errors.put(
                            subject,
                            new ElasticsearchException(
                                format(
                                    "profile [%s] matches search criteria but is not accessible to "
                                        + "the current subject with username [%s] and realm name [%s]",
                                    profileDocument.uid(),
                                    subject.getUser().principal(),
                                    subject.getRealm().getName()
                                )
                            )
                        );
                    }
                } else {
                    errors.put(
                        subject,
                        new ElasticsearchException(
                            format(
                                "multiple [%s] profiles [%s] found for user [%s] from realm [%s]%s",
                                hits.length,
                                Arrays.stream(hits)
                                    .map(SearchHit::getId)