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/profile/ProfileServiceTests.java
randomFrom(invalidFirstCharsOfLiteralUsername) + randomAlphaOfLengthBetween(0, 8),
            randomAlphaOfLengthBetween(1, 8) + randomFrom(invalidCharsOfLiteralUsername) + randomAlphaOfLengthBetween(0, 8)
        );
        final Authentication.RealmRef realmRef3 = realmRef2;
        final Authentication authentication3 = AuthenticationTestHelper.builder()
            .realm()
            .user(new User(invalidLiteralUsername))
            .realmRef(realmRef3)
            .build();
        final PlainActionFuture<Profile> future3 = new PlainActionFuture<>();
        service.activateProfile(authentication3, future3);
        final ElasticsearchException e3 = expectThrows(ElasticsearchException.class, future3::actionGet);
        assertThat(
            e3.getMessage(),
            containsString("Security domain [" + realmRef3.getDomain().name() + "] is configured to use literal username.")
        );
        assertThat(e3.getMessage(), containsString("The username must begin with an alphanumeric character"));
    }
    public void testShouldSkipUpdateForActivate() {
        // 
        final ProfileDocument profileDocument = randomProfileDocument(randomAlphaOfLength(40));
        final ProfileDocumentUser user = profileDocument.user();
        assertThat(
            profileService.shouldSkipUpdateForActivate(
                profileDocument,
                new ProfileDocument(
                    profileDocument.uid(),
                    profileDocument.enabled(),
                    profileDocument.lastSynchronized() + randomLongBetween(0, 29_999),
                    profileDocument.user(),