- Prevent negative measurements in Histogram Instruments so we
  can include monotonic sum in the data and export to prometheus.
- Remove explicit monotonicity argument to aggregators and always infer
  from instrument type.
  - Expand list of supported instruments for Sum aggregation with details
    on default aggregation.
  - Add note on handling `sum` in histogram for instruments that allow
    negative measurements.

Co-authored-by: Aaron Abbott <aaronabbott@google.com>