Add prometheus latency collector

[kate-osborn]

Proposed changes

Adds prometheus latency collector that calculates http upstream server latency in milliseconds using a prometheus histogram vector.
Latency metrics are published under the names:

• nginx_ingress_controller_upstream_server_response_latency_ms_bucket
• nginx_ingress_controller_upstream_server_response_latency_ms_sum
• nginx_ingress_controller_upstream_server_response_latency_ms_count

The metrics have the following labels: upstream, server, resource_namespace, resource_type, resource_name, service, pod_name, and code.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork

[pleshakov]

👍