ScanServer follow-on: TabletServerMetricsUtil changes

[dlmarion]

ScanServer can't use TabletServerMetricsUtil, so scanned count, query result count, and query byte count metrics are not being reported in the metrics. TabletServerMetricsUtil also uses TabletServer.getOnlineTablets to iterate over the Tablets and accumulate the metrics. ScanServer does not implement getOnlineTablets.

One approach to solving this would be to have a LongAdder (or similar) that could be used by each Tablet to increment. This avoids the iteration over the tablets to get the metrics as the LongAdder objects are updated by each Tablet in real-time.