[VIndex] Add metrics

[mhutchinson]

There are no metrics recorded nor exposed at the moment. We should have some basic ones:

• time since last output log checkpoint update (equivalently, time since last tree update)
• histogram of number of keys updated per batch
• histogram of number of keys returned from map function

Until this bug is closed, feel free to add comments to this requesting other metrics.

[mjl-]

Another suggestion: metrics about errors requesting from the inputlog/proxied sumdb.