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
new Profile.ProfileUser(
                            sampleDocumentParameter.username,
                            sampleDocumentParameter.roles,
                            "realm_name_1",
                            "domainA",
                            "foo@example.com",
                            "User Foo"
                        ),
                        Map.of(),
                        applicationData,
                        new Profile.VersionControl(1, 0)
                    )
                )
            );
            i++;
        }
    }
    public void testGetProfilesEmptyUids() {
        final PlainActionFuture<ResultsAndErrors<Profile>> future = new PlainActionFuture<>();
        profileService.getProfiles(List.of(), Set.of(), future);
        assertThat(future.actionGet().isEmpty(), is(true));
    }
    @SuppressWarnings("unchecked")
    public void testGetProfileSubjectsNoIndex() throws Exception {
        when(profileIndex.indexExists()).thenReturn(false);
        PlainActionFuture<ResultsAndErrors<Map.Entry<String, Subject>>> future = new PlainActionFuture<>();
        profileService.getProfileSubjects(randomList(1, 5, () -> randomAlphaOfLength(20)), future);
        ResultsAndErrors<Map.Entry<String, Subject>> resultsAndErrors = future.get();
        assertThat(resultsAndErrors.results().size(), is(0));
        assertThat(resultsAndErrors.errors().size(), is(0));