Prometheus / OpenTelemetry metrics

[bal-e]

Cascade aims to be transparent and easy to monitor. Offering metrics through standard systems like Prometheus or OpenTelemetry is key to that. To understand what kind of metrics we want to expose, see #298.

• Implement an HTTP endpoint for Prometheus within http_server.rs
• Implement some preliminary metrics based on Expand status information #298, by querying State and ZoneState
• Investigate adding a Grafana dashboard - see Investigate adding an example Grafana dashboard. #575
• Investigate adding support for / switching to OpenTelemetry - see Investigate adding support for / switching to OpenTelemetry. #576
• Document metrics (endpoint, and available metrics)

[ximon18]

@mozzieongit: Should we also track adding documentation about metrics, or at least about the metric functionality in general, not per se about every metric, as a pending TODO item for this GH issue?