cli: add streaming metrics to debug capture

[davemay99]

nomad operator debug currently saves metrics data once per capture interval. Because not all metrics are contained in each request, In many cases this results in missing data.

Proposal

Using the feature added in hashicorp/go-metrics#125, collect streaming metrics for the whole duration of the capture, rather than once per capture interval.

• Add streaming metrics RPC
• Add streaming metrics API
• Add streaming metrics to nomad operator metrics CLI command
• Add streaming metrics to nomad operator debug CLI command