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/core/src/main/java/org/elasticsearch/xpack/core/security/SecurityFeatureSetUsage.java
this.operatorPrivilegesUsage = operatorPrivilegesUsage;
        this.domainsUsage = domainsUsage;
    }
    @Override
    public Version getMinimalSupportedVersion() {
        return Version.V_7_0_0;
    }
    @Override
    public void writeTo(StreamOutput out) throws IOException {
        super.writeTo(out);
        out.writeGenericMap(realmsUsage);
        out.writeGenericMap(rolesStoreUsage);
        out.writeGenericMap(sslUsage);
        if (out.getVersion().onOrAfter(Version.V_7_2_0)) {
            out.writeGenericMap(tokenServiceUsage);
            out.writeGenericMap(apiKeyServiceUsage);
        }
        out.writeGenericMap(auditUsage);
        out.writeGenericMap(ipFilterUsage);
        out.writeGenericMap(anonymousUsage);
        out.writeGenericMap(roleMappingStoreUsage);
        if (out.getVersion().onOrAfter(Version.V_7_5_0)) {
            out.writeGenericMap(fips140Usage);
        }
        if (out.getVersion().onOrAfter(Version.V_7_11_0)) {
            out.writeGenericMap(operatorPrivilegesUsage);
        }
        if (out.getVersion().onOrAfter(Version.V_8_2_0)) {
            out.writeGenericMap(domainsUsage);
        }