Fix Prometheus latency histograms mis-scaled by 1000x

[wbarnha]

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Description

Fixes #260.

Every latency Histogram in FaustMetrics (events_runtime_latency, producer_send_latency, producer_error_send_latency, assign_latency, rebalance_done_consumer_latency, rebalance_done_latency, http_latency, consumer_commit_latency) was created with no explicit buckets=, so prometheus_client defaulted to its second-scale DEFAULT_BUCKETS (0.005–10.0).

Every one of these histograms is fed millisecond values via Monitor.ms_since() (see the various PrometheusMonitor.on_* methods). Real latency observations — typically tens to hundreds of milliseconds — mostly land in the +Inf overflow bucket instead of any meaningful bucket, silently mis-scaling every latency dashboard/alert built on Faust's Prometheus sensor by 1000x.

Fix

Add MS_LATENCY_BUCKETS, a millisecond-scale bucket tuple with the same shape as Histogram.DEFAULT_BUCKETS scaled x1000, and pass it explicitly to all 8 latency histograms.

Tests

• Parametrized test asserting every latency histogram's bucket boundaries equal MS_LATENCY_BUCKETS.
• A direct test that a 250ms observation lands in the 250ms bucket rather than the overflow bucket.
• Verified both fail without the fix (ImportError on the missing MS_LATENCY_BUCKETS symbol; KeyError on the 250.0 bucket boundary, since the old default buckets only had a 0.25 boundary) — confirming the tests aren't vacuous.
• flake8, black --check, isort --check-only clean.
• Full tests/unit/sensors suite passes (118 passed), excluding the pre-existing test_datadog.py errors caused by the datadog package not being installed in this sandbox — unrelated to this change.

🤖 Generated with Claude Code

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.15%. Comparing base (d95af52) to head (06d4cd3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #693   +/-   ##
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  Coverage   94.15%   94.15%           
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  Files         104      104           
  Lines       11136    11137    +1     
  Branches     1201     1201           
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+ Hits        10485    10486    +1     
  Misses        550      550           
  Partials      101      101           

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