Proposal

Related to prometheus/proposals#31

The histogram message type should have a new optional field that is a list of float64 values. These are to describe the custom bucket upper boundaries (excl. +Inf which is implicit). So basically encode what we currently put in the le label.

• Add custom_bounds field to remote write specification 2.0 : types.proto.

    // custom_bounds specify upper bounds for the buckets
    // with arbitrary widths for this histogram. The upper bounds are inclusive (following `le` label semantics),
    // however, the previous bound specifies the lower bound (exclusive), making the bucket count
    // non-cumulative (as opposed to `le` semantics in the past).
    // Bounds have to be monotonically increasing.
    // The last element is also a lower bound for the implicit +Inf bucket (overflow buckets).
    // The first element is the upper inclusive boundary for the first bucket,
    // which implicitly has a lower inclusive bound of -Inf.
    repeated double custom_bounds =  16;
  

• Update the note of the field schema to allow schema number 127. And also explain that in that case custom_bounds is taken into account for interpreting the custom buckets (stored in positive_spans, positive_deltas, positive_counts). Furthermore negative_spans, negative_deltas, negative_counts, zero_count and zero_threshold should be ignored.
• Test what happens to Prometheus if we send a remote write request with custom bucket definition and schema value of 127 - Prometheus should gracefully reject the sample currently (regardless of native histogram feature toggle!)
• Test the the field is truly optional and if not set, causes no problem.

This issue should target remote write 2.0.