Reject repeated histogram bucket bounds

[chrikrah]

A NaN bucket bound passes Histogram._prepare_buckets (prometheus_client/metrics.py:642), because buckets != sorted(buckets) never fires on it. The histogram then exposes an le="NaN" series that no observation can land in. This change requires strictly increasing bounds: [0.1, nan, 1.0] and a repeated bound such as [0.1, 0.5, 0.5, 1.0] now raise ValueError at construction. No issue exists for this.

Before, at 9cd073c:

# Python 3.12.3; buckets=[0.1, float('nan'), 1.0], observe(0.05), observe(0.7)
$ PYTHONPATH=. python3 nan_bucket.py | grep _bucket
h_bucket{le="0.1"} 1.0
h_bucket{le="NaN"} 1.0
h_bucket{le="1.0"} 2.0
h_bucket{le="+Inf"} 2.0

The repeated bound is the part to decide. Prometheus ingests it today, since both le="0.5" series always carry the same value. Raising there buys consistency with client_golang ("histogram buckets must be in increasing order", prometheus/histogram.go:590), not ingestion safety. Code that passes one today gets a ValueError with this change. A lone [nan] still passes, as the check compares pairs.

Verification

# Python 3.14.7
$ python -m pytest -q tests
428 passed, 1 skipped in 10.21s
# base 9cd073c: 428 passed, 1 skipped (the new cases are assertions inside test_setting_buckets)
$ git stash -- prometheus_client/metrics.py && python -m pytest -q tests/test_core.py   # fix reverted, test kept
FAILED tests/test_core.py::TestHistogram::test_setting_buckets - AssertionError: ValueError not raised by Histogram
1 failed, 113 passed in 3.73s
$ flake8 prometheus_client/ tests/ && isort --check prometheus_client/ tests/   # flake8 7.4.1, isort 5.10.1

This touches the same function as #1104. I will rebase onto whichever lands first.

@csmarchbanks, should a repeated bound raise as here, or be dropped silently as client_java does?