Improve resource manager observability

[sukunrt]

We should provide more inspection tools for users to make it easier for users to inspect resource manager usage. I have 3 specific items in mind.

• Provide a way to print resource usage: Add a way to print resource limits and current consumption #3193
• Add prometheus metrics for the configured limits so these can be added to dashboards for convenient checking of the upper bound.
• Log the configured rcmgr limits on startup
• Improve the documentation for Limits
  • What's a PartialLimitConfig
  • What's a ScalingLimitConfig
  • What's a default LimitVal which depends on context
  • Move a lot of the README stuff to the actual objects so godoc can pick it up
  • Add many more examples for common things people want
• Simplify the API for adjusting the default resources allocated. By default, an 1/8 of the resources are allocated. It should be easy enough to adjust this multiplier.

cc @2color