Enhance APIcast policy with more Prometheus metrics

Per conversation here creating a new issue to enhance the APIcast policy with more Prometheus metrics.

Metrics to be added for 2.4

• 3scale-auth status codes: Total, 2xx, 4xx, 5xx
• Upstream status codes: Total, 2xx, 4xx, 5xx
• Request time:
  must full end to end latency
  must upstream latency
  optional breakdown of latency in APIcast pre- and post-request connections (per Joaquim): Read, Write, Wait, Open

See gh issue

Needs documentation.