Adjust gorouter prom-scraper "Histogram" metrics to "Gauge"

[plowin]

Proposed Change

As an operator
I want gorouter to emit metrics in an automatically scalable format
So that I can use the data without adjustments on operator-side.

• Prometheus metric exports have been introduced in https://github.com/cloudfoundry/gorouter/pull/466/files
• The Histogram metrics in this struct should be adjusted to Gauge as it is more flexible to visualize spikes and outliers: https://github.com/cloudfoundry/gorouter/blob/b5be7589592b9cf21f69dd74055bdcc77157e369/metrics_prometheus/metrics.go#L17-L44
• The metrics are active if enable(_prom)_scraper is set to true and a prometheus port is defined
• Related discussion on Slack, an incompatible can be considered here

Acceptance criteria

Scenario: Gorouter emits Histogram metrics in a new format
Given an active prometheus scraper and configured port
When building dashboards on top of the data
And visualize e.g. RouteRegistrationLatency
Then I do not need to worry about the buckets and get a usable graph without postprocessing the data

Related links

No response