code: public String asJSON() {
      Gson gson = new Gson();
  
      clearExpired();
  
      Cache jsonCache = new Cache();
      for (ExpiringService svc : cache.values()) {
        jsonCache.cache.add(svc.getService());
      }
  
      String json = gson.toJson(jsonCache);
  
      return json;
    }
docs: Return the JSON string of the response cache. Leverage the Gson nature of
  the domain wireline classes
  
  @return the JSON string