Prometheus exporter: sanitization tweaks

[dashpole]

I would recommend adjusting the prefix logic during sanitization. Prometheus/OpenMetrics allow a metric to start with _, so there is no need to prefix a _ with key here. For metrics that start with a number, I would recommend prefixing with _ instead of key_ to match other implementations here.

[dashpole]

Actually, reading this again https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#name, I don't think this advice is correct.