The NiFi UI has a Summary page from the top-right menu, as well as stats on each processor by right-clicking and selecting Status History. Both of those views show things like bytes read/written, flow files in/out, etc.
If you want to track this information somewhere else, you can implement a custom ReportingTask to send this information somewhere. This is how Ambari is able to display the stats/graphs, there is an AmbariReportingTask:
https://github.com/apache/nifi/blob/master/nifi-nar-bundles/nifi-ambari-bundle/nifi-ambari-reporting...