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
private static final Logger logger = LogManager.getLogger(ProfileService.class);
    private static final String DOC_ID_PREFIX = "profile_";
    private static final BackoffPolicy DEFAULT_BACKOFF = BackoffPolicy.exponentialBackoff();
    private static final int DIFFERENTIATOR_UPPER_LIMIT = 9;
    private static final long ACTIVATE_INTERVAL_IN_MS = TimeValue.timeValueSeconds(30).millis();
    private final Settings settings;
    private final Clock clock;
    private final Client client;
    private final SecurityIndexManager profileIndex;
    private final ClusterService clusterService;
    private final Function<String, DomainConfig> domainConfigLookup;
    private final ThreadPool threadPool;
    public ProfileService(
        Settings settings,
        Clock clock,
        Client client,
        SecurityIndexManager profileIndex,
        ClusterService clusterService,
        Function<String, DomainConfig> domainConfigLookup,
        ThreadPool threadPool
    ) {
        this.settings = settings;
        this.clock = clock;
        this.client = client;
        this.profileIndex = profileIndex;
        this.clusterService = clusterService;
        this.domainConfigLookup = domainConfigLookup;
        this.threadPool = threadPool;