Feature Request: Additional table and index metrics

[rafer]

Feature Description

Add metrics to vttablet for the following:

• TableRows (by table) - The estimated number of rows in the table
• TableClusteredIndexSize (by table) - The size of the clustered index (i.e. the size of the "row data")
• IndexCardinality (by index) - The estimated number of unique values in the index
• IndexBytes (by index) - The size of the index

Use Case(s)

These metrics are handy to have to keep an eye on tables/indexes over time. These can be loaded from mysql system tables on the tablets and published as metrics from vttablet.