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/Security.java
import org.elasticsearch.watcher.ResourceWatcherService;
import org.elasticsearch.xcontent.NamedXContentRegistry;
import org.elasticsearch.xpack.core.XPackField;
import org.elasticsearch.xpack.core.XPackPlugin;
import org.elasticsearch.xpack.core.XPackSettings;
import org.elasticsearch.xpack.core.action.XPackInfoFeatureAction;
import org.elasticsearch.xpack.core.action.XPackUsageFeatureAction;
import org.elasticsearch.xpack.core.security.SecurityContext;
import org.elasticsearch.xpack.core.security.SecurityExtension;
import org.elasticsearch.xpack.core.security.SecurityField;
import org.elasticsearch.xpack.core.security.SecuritySettings;
import org.elasticsearch.xpack.core.security.action.ClearSecurityCacheAction;
import org.elasticsearch.xpack.core.security.action.DelegatePkiAuthenticationAction;
import org.elasticsearch.xpack.core.security.action.apikey.BulkUpdateApiKeyAction;
import org.elasticsearch.xpack.core.security.action.apikey.CreateApiKeyAction;
import org.elasticsearch.xpack.core.security.action.apikey.GetApiKeyAction;
import org.elasticsearch.xpack.core.security.action.apikey.GrantApiKeyAction;
import org.elasticsearch.xpack.core.security.action.apikey.InvalidateApiKeyAction;
import org.elasticsearch.xpack.core.security.action.apikey.QueryApiKeyAction;
import org.elasticsearch.xpack.core.security.action.apikey.UpdateApiKeyAction;
import org.elasticsearch.xpack.core.security.action.enrollment.KibanaEnrollmentAction;
import org.elasticsearch.xpack.core.security.action.enrollment.NodeEnrollmentAction;
import org.elasticsearch.xpack.core.security.action.oidc.OpenIdConnectAuthenticateAction;
import org.elasticsearch.xpack.core.security.action.oidc.OpenIdConnectLogoutAction;
import org.elasticsearch.xpack.core.security.action.oidc.OpenIdConnectPrepareAuthenticationAction;
import org.elasticsearch.xpack.core.security.action.privilege.ClearPrivilegesCacheAction;
import org.elasticsearch.xpack.core.security.action.privilege.DeletePrivilegesAction;
import org.elasticsearch.xpack.core.security.action.privilege.GetBuiltinPrivilegesAction;
import org.elasticsearch.xpack.core.security.action.privilege.GetPrivilegesAction;
import org.elasticsearch.xpack.core.security.action.privilege.PutPrivilegesAction;