[stacked 1/5] metrics: cleanup metrics registration, collection and gathering.

[klihub]

This patch series refactors our metrics collection code, moving most of the code to handle metrics registration, collection, and gathering into a single package in pkg/metrics. In particular, this series

• consolidates all common metrics-handling code in pkg/metrics
• removes metrics polling scattered around in resmgr, resmgr/{metrics,policycollector}
• moves policy-specific metrics collection/polling to the policy package itself
• switches from opencensus to promhttp for metrics exporting in pkg/instrumentation

[klihub]

Looks great!

@klihub, would you prefer to address my nits & supernits (to the extent you find reasonable) now or later? I'm fine merging this as is.

Let me take a look at them and see how many I could address right now.

[klihub]

Looks great! @klihub, would you prefer to address my nits & supernits (to the extent you find reasonable) now or later? I'm fine merging this as is.

Let me take a look at them and see how many I could address right now.

@askervin I tried to address all your comments.