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/main/java/org/elasticsearch/xpack/security/profile/ProfileService.java
&& newProfileDocument.enabled() == currentProfileDocument.enabled()
            && newProfileDocument.lastSynchronized() - currentProfileDocument.lastSynchronized() < ACTIVATE_INTERVAL_IN_MS) {
            return true;
        }
        return false;
    }
    private UpdateRequest buildUpdateRequest(String uid, XContentBuilder builder, RefreshPolicy refreshPolicy) {
        return buildUpdateRequest(uid, builder, refreshPolicy, -1, -1);
    }
    private UpdateRequest buildUpdateRequest(
        String uid,
        XContentBuilder builder,
        RefreshPolicy refreshPolicy,
        long ifPrimaryTerm,
        long ifSeqNo
    ) {
        final String docId = uidToDocId(uid);
        final UpdateRequestBuilder updateRequestBuilder = client.prepareUpdate(SECURITY_PROFILE_ALIAS, docId)
            .setDoc(builder)
            .setRefreshPolicy(refreshPolicy);
        if (ifPrimaryTerm >= 0) {
            updateRequestBuilder.setIfPrimaryTerm(ifPrimaryTerm);
        }
        if (ifSeqNo >= 0) {
            updateRequestBuilder.setIfSeqNo(ifSeqNo);
        }
        return updateRequestBuilder.request();
    }
    
    void doUpdate(UpdateRequest updateRequest, ActionListener<UpdateResponse> listener) {