protobuf: replace/augment OpenMetrics protobuf with Prometheus format

[howardjohn]

Currently, this library supports OpenMetrics proto encoding. However, it does not support Prometheus protobuf encoding.

This seems reasonable on the surface as it mirrors the text encoding, but (IIUC) developments on the Prometheus side suggest Prometheus protobuf encoding should be favored.

See prometheus/OpenMetrics#296 (comment) and prometheus/OpenMetrics#256 (comment) for context.

I am open to contributing this feature, though would appreciate some guidance on whether we should replace OM proto or add an alternative encoder.

[mxinden]

Thank you for the context.

I think long term we should do whatever client_go does. I am fine replacing the current OM proto with Prometheus proto in one step.