OpenTelemetry Collector uses metadata.yaml files
Prometheus exporters define metrics in Go code via prometheus.NewDesc()
Python instrumentation uses decorators and meter APIs
Some sources just have documentation (if you're lucky)
You can see the details of how the registry was built on the repo - https://github.com/base-14/metric-library . the current setup scans through many sources and has details for 3700+ metrics. The scan runs every night(/day depending on where you live)
You’re experiencing observability theatre, the expensive illusion of system visibility without its substance.