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
resourceWatcherService,
            nativeRoleMappingStore
        );
        Map<String, Realm.Factory> realmFactories = new HashMap<>(
            InternalRealms.getFactories(
                threadPool,
                settings,
                resourceWatcherService,
                getSslService(),
                nativeUsersStore,
                nativeRoleMappingStore,
                systemIndices.getMainIndexManager()
            )
        );
        for (SecurityExtension extension : securityExtensions) {
            Map<String, Realm.Factory> newRealms = extension.getRealms(extensionComponents);
            for (Map.Entry<String, Realm.Factory> entry : newRealms.entrySet()) {
                if (realmFactories.put(entry.getKey(), entry.getValue()) != null) {
                    throw new IllegalArgumentException("Realm type [" + entry.getKey() + "] is already registered");
                }
            }
        }
        final Realms realms = new Realms(
            settings,
            environment,
            realmFactories,
            getLicenseState(),
            threadPool.getThreadContext(),
            reservedRealm
        );