docs: public static HystrixThreadPoolMetrics getInstance(HystrixThreadPoolKey key, ThreadPoolExecutor threadPool, HystrixThreadPoolProperties properties) {
          // attempt to retrieve from cache first
          HystrixThreadPoolMetrics threadPoolMetrics = metrics.get(key.name());
          if (threadPoolMetrics != null) {
              return threadPoolMetrics;
          } else {
              synchronized (HystrixThreadPoolMetrics.class) {
                  HystrixThreadPoolMetrics existingMetrics = metrics.get(key.name());
                  if (existingMetrics != null) {
                      return existingMetrics;
                  } else {
                      HystrixThreadPoolMetrics newThreadPoolMetrics = new HystrixThreadPoolMetrics(key, threadPool, properties);
                      metrics.putIfAbsent(key.name(), newThreadPoolMetrics);
                      return newThreadPoolMetrics;
                  }
              }
          }
      }