Fix #214: skip end-offset metric when highwater is unknown (None)

[wbarnha]

What

During a rebalance the consumer could crash _drain_messages with:

TypeError: float() argument must be a string or a number, not 'NoneType'
  File ".../faust/transport/consumer.py", line 715, in getmany
    self.app.monitor.track_tp_end_offset(tp, highwater_mark)
  File ".../faust/sensors/prometheus.py", line 496, in track_tp_end_offset
    self._metrics.topic_partition_end_offset.labels(...).set(float(value))

Fixes #214.

Why

Consumer.highwater(tp) legitimately returns None during a rebalance, before the partition's end offset is known. getmany() passed that value straight into monitor.track_tp_end_offset(tp, highwater_mark). The base Monitor stores it harmlessly, but the PrometheusMonitor / DatadogMonitor / StatsdMonitor implementations call float(offset), so a None raises TypeError and takes down the drain loop (the AeroSpike/rebalance framing in the report is incidental — any storage backend can hit this).

How

Guard the call in getmany() so the end-offset metric is only tracked once the highwater is known. Fixing it at the single source point covers every metric sensor at once, rather than patching float() in each sensor separately, and is semantically correct — there is no meaningful end offset to record yet.

Test

Added two tests in tests/unit/transport/test_consumer.py:

• test_getmany__highwater_none_not_tracked — highwater() returns None; asserts messages still flow and track_tp_end_offset is not called (verified to fail before the fix).
• test_getmany__highwater_tracked — highwater() returns an int; asserts track_tp_end_offset is called with it.

Full tests/unit/transport/test_consumer.py passes (115 passed, 2 skipped).

🤖 Generated with Claude Code

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.15%. Comparing base (3073eb9) to head (e6871b2).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #703   +/-   ##
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  Coverage   94.14%   94.15%           
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  Files         104      104           
  Lines       11136    11137    +1     
  Branches     1201     1202    +1     
=======================================
+ Hits        10484    10486    +2     
+ Misses        551      550    -1     
  Partials      101      101           

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