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/test/java/org/elasticsearch/xpack/security/SecurityInfoTransportActionTests.java
XContentSource source = getXContentSource(usage);
            if (enabled) {
                for (int i = 0; i < 5; i++) {
                    assertThat(source.getValue("realms.type" + i + ".key1"), contains("value" + i));
                    assertThat(source.getValue("realms.type" + i + ".key2"), contains(i));
                    assertThat(source.getValue("realms.type" + i + ".key3"), contains(i % 2 == 0));
                }
                
                assertThat(source.getValue("ssl.http.enabled"), is(httpSSLEnabled));
                assertThat(source.getValue("ssl.transport.enabled"), is(transportSSLEnabled));
                
                assertThat(source.getValue("token_service.enabled"), is(tokenServiceEnabled));
                
                assertThat(source.getValue("api_key_service.enabled"), is(apiKeyServiceEnabled));
                
                assertThat(source.getValue("audit.enabled"), is(auditingEnabled));
                if (auditingEnabled) {
                    assertThat(source.getValue("audit.outputs"), contains(LoggingAuditTrail.NAME));
                } else {
                    assertThat(source.getValue("audit.outputs"), is(nullValue()));
                }
                
                assertThat(source.getValue("ipfilter.http.enabled"), is(httpIpFilterEnabled));
                assertThat(source.getValue("ipfilter.transport.enabled"), is(transportIPFilterEnabled));
                
                if (rolesStoreEnabled) {
                    assertThat(source.getValue("roles.count"), is(1));
                } else {
                    assertThat(((Map) source.getValue("roles")).isEmpty(), is(true));
                }