support scan metrics

[TanUkkii007]

Add scan metrics functions in the Scanner.

Following scan metrics are added. Those lists are part of the metrics from the official HBase client.

• server-side metrics
  • ROWS_SCANNED
  • ROWS_FILTERED
• client-side metrics
  • RPC_CALLS
  • MILLIS_BETWEEN_NEXTS
  • NOT_SERVING_REGION_EXCEPTION
  • BYTES_IN_RESULTS
  • REGIONS_SCANNED
  • RPC_RETRIES_METRIC_NAME

Integration tests are also included and checked with HBase 1.2.6.

Known issues

• It seems the count of RPCs is different among HBase versions. I tested with HBase 0.94 and the number of RPCs is higher than 1.2.6. This fails the integration tests against 0.94.
• Server-side metrics is supported for HBase 0.95 and higher. Tests about server-side metrics are skipped if the server-side metrics show zero value.

[TanUkkii007]

BTW, recently I talked about migration to asynchbase. Scan metrics I implemented in this PR are shown in my slides.
https://www.slideshare.net/TanUkkii/nonblocking-io-to-tame-distributed-systems-how-and-why-chatwork-uses-asynchbase