[haeganm]

Histogram leaves out _sum when its lowest bucket is negative. that's been the case since #496, it's what OpenMetrics 1.0 says ("Negative threshold buckets MAY be used, but then the Histogram MetricPoint MUST NOT contain a sum value"), and it was confirmed as intended in #552 and #782. MultiProcessCollector builds histogram samples on its own and never checks for this, so in multiprocess mode the same histogram still has a _sum:

# Histogram('temp_delta', 'help', buckets=[-1, 0, 1]).observe(-0.5), multiprocess
temp_delta_sum -0.5
temp_delta_bucket{le="-1.0"} 0.0
temp_delta_bucket{le="0.0"} 1.0
temp_delta_bucket{le="1.0"} 1.0
temp_delta_bucket{le="+Inf"} 1.0
temp_delta_count 1.0

single process gives the same lines without temp_delta_sum.

this drops _sum on the accumulate path when the lowest bucket is negative, so both modes match. merge(files, accumulate=False) still returns the raw value, since that's for writing merged values back to files. I also added a line to the histogram docs, since they list _sum as always there.

heads up, this means multiprocess users with negative buckets lose _sum. and OpenMetrics 2.0 goes the other way (every histogram MUST have count and sum, and negative thresholds are allowed). if you'd rather keep _sum and change single process instead, happy to flip it.

test_histogram_negative_buckets fails on master (AssertionError: 1.5 is not None) and passes here. full suite passes on 3.10 (Linux) and 3.12 (Windows), and flake8, isort and mypy are clean.

@csmarchbanks