Create a dummy metric implementation in ConnectionImplementation.

This Jira is for branch-2 only.

If CLIENT_SIDE_METRICS_ENABLED_KEY conf is set to true, then we initialize metrics to MetricsConnection, otherwise it is set to null.

      if (conf.getBoolean(CLIENT_SIDE_METRICS_ENABLED_KEY, false)) {        
          this.metricsScope = MetricsConnection.getScope(conf, clusterId, this);        
          this.metrics = MetricsConnection.getMetricsConnection(this.metricsScope, this::getBatchPool, this::getMetaLookupPool);
      } else {
          this.metrics = null;      
      }

Whenever we want to update metrics, we always do a null check. We can improve this by creating a dummy metrics object and have an empty implementation. When we want to populate the metrics, we can check if metrics is an instance of dummy metrics.