Provide zilla metrics command to report current values locally

[jfallows]

Note: requires design #109.

Zilla current has a zilla load command in the incubator that reports on the following metrics for each binding.

• tx.opens, rx.opens
• tx.closes, rx.closes
• tx.errors, rx.errors
• tx.bytes, rx.bytes

Rename zilla load command to zilla metrics command and report on using the new local storage structures for metrics.

Given that the list of metrics is now extensible, a tabular layout is probably no longer the best option.

Note that while the above metrics are all counters, there are general requirements to support gauge and histogram metrics as well, which affects the information presented for each metric.

Promote zilla metrics out of the incubator to the runtime.