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
private synchronized SharedGroupFactory getNettySharedGroupFactory(Settings settings) {
        if (sharedGroupFactory.get() != null) {
            assert sharedGroupFactory.get().getSettings().equals(settings) : "Different settings than originally provided";
            return sharedGroupFactory.get();
        } else {
            sharedGroupFactory.set(new SharedGroupFactory(settings));
            return sharedGroupFactory.get();
        }
    }
    @Override
    public Collection<SystemIndexDescriptor> getSystemIndexDescriptors(Settings settings) {
        return systemIndices.getSystemIndexDescriptors();
    }
    @Override
    public String getFeatureName() {
        return "security";
    }
    @Override
    public String getFeatureDescription() {
        return "Manages configuration for Security features, such as users and roles";
    }
    @Override
    public CheckedBiConsumer<ShardSearchRequest, StreamOutput, IOException> getRequestCacheKeyDifferentiator() {
        if (enabled == false) {
            return null;
        }
        return new DlsFlsRequestCacheDifferentiator(getLicenseState(), securityContext, scriptServiceReference);
    }
}