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
if (missingProfileUids.contains(item.id().substring("profile_".length()))) {
                    GetResponse missingResponse = mock(GetResponse.class);
                    when(missingResponse.isExists()).thenReturn(false);
                    when(missingResponse.getId()).thenReturn(item.id());
                    responses.add(new MultiGetItemResponse(missingResponse, null));
                } else {
                    String source = getSampleProfileDocumentSource(
                        item.id().substring("profile_".length()),
                        "foo_username_" + item.id().substring("profile_".length()),
                        List.of("foo_role_" + item.id().substring("profile_".length())),
                        Instant.now().toEpochMilli()
                    );
                    GetResult existingResult = new GetResult(
                        SECURITY_PROFILE_ALIAS,
                        item.id(),
                        0,
                        1,
                        1,
                        true,
                        new BytesArray(source),
                        emptyMap(),
                        emptyMap()
                    );
                    responses.add(new MultiGetItemResponse(new GetResponse(existingResult), null));
                }
            }
            final ActionListener<MultiGetResponse> listener = (ActionListener<MultiGetResponse>) invocation.getArguments()[2];
            listener.onResponse(new MultiGetResponse(responses.toArray(MultiGetItemResponse[]::new)));
            return null;
        }).when(client).execute(eq(MultiGetAction.INSTANCE), any(MultiGetRequest.class), anyActionListener());