* clean prometheus metrics
- add new histogram metrics with consistent names
- deprecate summary metrics with inconsistent names
* update prometheus metrics tests
* remove ingress_upstream_header_seconds metric
It hasn't been released so it is safe. Use header_duration_seconds metric.
* add documentation on prometheus metrics
This commit introduces a backwards compatible command line option
--report-status-classes which will enable reporting response status classes
(2xx, 3xx..) instead of status codes in exported metrics.
* fix inconsistent-label-cardinality
for prometheus metrics: nginx_ingress_controller_requests
* add host to collectorLabels only if metricsPerHost is true
When serving many hosts from one nginx, the metrics may become too
numerous for Prometheus. Add a flag to disable the host label, so
that metrics are totalled across all hosts.
