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
listener::onFailure
                    )
                );
            });
        });
    }
    private static SubjectSearchResultsAndErrors<VersionedDocument> convertSubjectMultiSearchResponse(
        MultiSearchResponse multiSearchResponse,
        List<Subject> subjects
    ) throws IOException {
        final MultiSearchResponse.Item[] items = multiSearchResponse.getResponses();
        assert items.length == subjects.size() : "size of responses does not match size of subjects";
        final List<Tuple<Subject, VersionedDocument>> versionedDocs = new ArrayList<>(items.length);
        final Map<Subject, Exception> errors = new HashMap<>();
        for (int i = 0; i < items.length; i++) {
            final MultiSearchResponse.Item item = items[i];
            final Subject subject = subjects.get(i);
            if (item.isFailure()) {
                errors.put(subject, item.getFailure());
            } else {
                final SearchHits searchHits = item.getResponse().getHits();
                final SearchHit[] hits = searchHits.getHits();
                if (hits.length < 1) {
                    logger.debug(
                        "profile does not exist for username [{}] and realm name [{}]",
                        subject.getUser().principal(),
                        subject.getRealm().getName()
                    );
                    versionedDocs.add(new Tuple<>(subject, null));
                } else if (hits.length == 1) {