Histograms: Port the protobuf changes to the OpenMetrics protobuf spec

[beorn7]

The changes made here to the protobuf spec to accommodate Native Histograms (AKA Sparse Histograms) only affect the old Prometheus protobuf format, which is hardly used (and had to be brought back in prometheus/prometheus just for the histogram experiment).

However, it is likely that the changes will easily translate to the OpenMetrics protobuf spec, see https://github.com/OpenObservability/OpenMetrics/blob/main/proto/openmetrics_data_model.proto .

This issue is about exploring that possibility. If it looks feasible, open a PR against https://github.com/OpenObservability/OpenMetrics , which would be an easy way to bring Native Histograms to OpenMetrics without first solving the problems around representing Native Histograms in the OM text format (cf. prometheus/OpenMetrics#237 ).

Corollary: prometheus/prometheus should then also support the OpenMetrics protobuf format. With a bit of luck, this will be easy to implement based on the protobuf parsing code created for the histogram experiment, see first paragraph above).

[beorn7]

Note that this issue doesn't block merging the sparsehistogram branch of this repo into the master branch.

[beorn7]

prometheus/OpenMetrics#256 is a draft for how the OpenMetrics protobuf could look like.

[beorn7]

prometheus/OpenMetrics#256 is not a draft anymore, and it seems to be correct as far as I can know. The reason it isn't merged is that OpenMetrics has to come to terms how to manage this change WRT version numbers etc.

Therefore, I'm closing this issue, as the work is done from prometheus/client_model's POV.