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/core/src/main/java/org/elasticsearch/xpack/core/security/SecurityFeatureSetUsage.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.core.security;
import org.elasticsearch.Version;
import org.elasticsearch.common.io.stream.StreamInput;
import org.elasticsearch.common.io.stream.StreamOutput;
import org.elasticsearch.xcontent.XContentBuilder;
import org.elasticsearch.xpack.core.XPackFeatureSet;
import org.elasticsearch.xpack.core.XPackField;
import java.io.IOException;
import java.util.Collections;
import java.util.Map;
public class SecurityFeatureSetUsage extends XPackFeatureSet.Usage {
    private static final String REALMS_XFIELD = "realms";
    private static final String ROLES_XFIELD = "roles";
    private static final String ROLE_MAPPING_XFIELD = "role_mapping";
    private static final String SSL_XFIELD = "ssl";
    private static final String TOKEN_SERVICE_XFIELD = "token_service";
    private static final String API_KEY_SERVICE_XFIELD = "api_key_service";
    private static final String AUDIT_XFIELD = "audit";
    private static final String IP_FILTER_XFIELD = "ipfilter";
    private static final String ANONYMOUS_XFIELD = "anonymous";
    private static final String FIPS_140_XFIELD = "fips_140";
    private static final String OPERATOR_PRIVILEGES_XFIELD = XPackField.OPERATOR_PRIVILEGES;
    private static final String DOMAINS_XFIELD = "domains";
    private Map<String, Object> realmsUsage;