public void initializeNotFoundCacheMetrics(Cache cache) {
    metricRegistry.gauge(MetricRegistry.name(NonBlockingRouter.class, "BlobsNotFoundCacheHitRate"),
        () -> (Gauge<Double>) () -> cache.stats().hitRate());
  }
Initializes a {@link Gauge} metric to monitor the cache hit rate for {@link Cache} keeping track of blobs
which known not to be present in Ambry.
@param cache which keeps track of blobs not found recently.
ambry-router/src/main/java/com/github/ambry/router/NonBlockingRouterMetrics.java