fix: order Prometheus histogram metrics correctly

[robin-ede]

What does this PR address?

Prometheus histograms must list every _bucket before their _count and _sum.
Out-of-order output breaks parsers like fluent-bit.
This patch guarantees the correct order for all BentoML histogram metrics.

Changes

• PrometheusClient
  • Add _fix_histogram_ordering()
    • Sorts buckets by le, then appends _count, _sum.
    • Transparent in single- and multi-process modes..

Checklist

• [x] Conventional commit title.
• [x] pre-commit run -a passes.
• [x] No doc updates needed.
• [x] Tests run.

Fixes #5386

[frostming]

What caused us to have to use this dirty method to fix it? How does it produce output in wrong order?