docs: public static HystrixCollapserMetrics getInstance(HystrixCollapserKey key, HystrixCollapserProperties properties) {
          // attempt to retrieve from cache first
          HystrixCollapserMetrics collapserMetrics = metrics.get(key.name());
          if (collapserMetrics != null) {
              return collapserMetrics;
          }
          // it doesn't exist so we need to create it
          collapserMetrics = new HystrixCollapserMetrics(key, properties);
          // attempt to store it (race other threads)
          HystrixCollapserMetrics existing = metrics.putIfAbsent(key.name(), collapserMetrics);
          if (existing == null) {
              // we won the thread-race to store the instance we created
              return collapserMetrics;
          } else {
              // we lost so return 'existing' and let the one we created be garbage collected
              return existing;
          }
      }