Metrics SDK defaults Histogram instruments to report monotonic sums, but allows negative measurements.

[jsuereth]

Currently, the APi does not specify that histograms record only non-negative instruments, but the DataModel requires the sum field to be a (positive) monotonic sum. These are incompatible with each other and limit prometheus compatibility.

I think we have a few solutions to this issue:

• Update SDK specification for Histogram aggregation to denote that it cannot produce the sum field with Histogram instrument (only Counter instrument).
• Update API specification to disallow negative measurements on Histogram instrument. Primary use case for histograms is latency and we don't expect many negative measurements for this key use case.
• Create a new "Timer" instrument for the latency use case that doesn't support negative measurements and allow Histogram instrument to record negative measurements.

If we don't resolve this, histograms being exported to Prometheus will need explicit View configuration to denote monotonicity on histograms.