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/qa/profile/src/javaRestTest/java/org/elasticsearch/xpack/security/profile/ProfileIT.java
final Request indexRequest = new Request("PUT", ".security-profile/_doc/profile_" + uid);
            indexRequest.setJsonEntity(source);
            indexRequest.addParameter("refresh", "wait_for");
            indexRequest.setOptions(
                expectWarnings(
                    "this request accesses system indices: [.security-profile-8], but in a future major version, "
                        + "direct access to system indices will be prevented by default"
                )
            );
            assertOK(adminClient().performRequest(indexRequest));
        }
        
        final Map<String, Object> responseMap1 = doGetProfiles(uids, null);
        @SuppressWarnings("unchecked")
        final List<Map<String, Object>> profiles1 = (List<Map<String, Object>>) responseMap1.get("profiles");
        assertThat(profiles1.size(), equalTo(uids.size()));
        IntStream.range(0, profiles1.size()).forEach(i -> {
            final Map<String, Object> profileMap = profiles1.get(i);
            final String uid = uids.get(i);
            assertThat(profileMap.get("uid"), equalTo(uid));
            assertThat(castToMap(profileMap.get("user")).get("username"), equalTo(uid));
            assertThat(castToMap(profileMap.get("user")).get("realm_name"), equalTo("realm_name_1"));
            assertThat(castToMap(profileMap.get("user")).get("realm_domain"), equalTo("domainA"));
            assertThat(castToMap(profileMap.get("data")), anEmptyMap());
        });
        
        final Map<String, Object> responseMap2 = doGetProfiles(uids, "app1");
        @SuppressWarnings("unchecked")
        final List<Map<String, Object>> profiles2 = (List<Map<String, Object>>) responseMap2.get("profiles");
        assertThat(profiles2.size(), equalTo(uids.size()));