Emit metrics for block caches

[keith-turner]

Metrics need to be emitted for the scan block caches in order to tune them. Currently a message is logged w/ information about them but metrics would be much better. Metrics can be emitted based on this information. It should be possible to compute the hit ratio over time of each block cache using the metrics data.

[DomGarguilo]

I can start looking into this

[DomGarguilo]

It should be possible to compute the hit ratio over time of each block cache using the metrics data.

@keith-turner Is this something that you think should be done before or after the metric has been emitted? I am guessing we do not want to do any math on the block cache hits and cache requests and just emit them as they are, and then users can do the calculations as they want. Please correct me if I am wrong in assuming this.

[DomGarguilo]

Closing via #4518