Add Per-Table metrics back

We used to have per-table metrics, but it was removed in some restructuring. We have per-region metrics, and per-regionserver metrics, but nothing in between.

For majority of users, per-region is too granular, they are mostly interested in table level aggregates. This is especially useful in multi-tenant cases where a table's disk usage, number of requests, etc can be made much more visible.

In this jira, we'll add the basic infrastructure to add a single (or a few) per-table metrics. Than we can improve on that by adding remaining metrics from the region server level.

The plan is to NOT aggregate per-table metrics at master for now. Just aggregation of per-region metrics at the per-table level for every regionserver.