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 List<String> labelValues = label.v2();
                query.should(QueryBuilders.termsQuery("user_profile.labels." + label.v1(), labelValues));
            }
            query.minimumShouldMatch(0);
        }
        final SearchRequest searchRequest = client.prepareSearch(SECURITY_PROFILE_ALIAS)
            .setQuery(query)
            .setSize(request.getSize())
            .addSort("_score", SortOrder.DESC)
            .addSort("user_profile.last_synchronized", SortOrder.DESC)
            .request();
        searchRequest.setParentTask(parentTaskId);
        return searchRequest;
    }
    private void getVersionedDocument(String uid, ActionListener<VersionedDocument> listener) {
        tryFreezeAndCheckIndex(listener).ifPresent(frozenProfileIndex -> {
            final GetRequest getRequest = new GetRequest(SECURITY_PROFILE_ALIAS, uidToDocId(uid));
            frozenProfileIndex.checkIndexVersionThenExecute(
                listener::onFailure,
                () -> executeAsyncWithOrigin(client, getActionOrigin(), GetAction.INSTANCE, getRequest, ActionListener.wrap(response -> {
                    if (false == response.isExists()) {
                        logger.debug("profile with uid [{}] does not exist", uid);
                        listener.onResponse(null);
                        return;
                    }
                    listener.onResponse(
                        new VersionedDocument(
                            buildProfileDocument(response.getSourceAsBytesRef()),
                            response.getPrimaryTerm(),
                            response.getSeqNo()