Make opentracing an optional dependency (faust[opentracing])

[wbarnha]

Description

opentracing is archived/EOL (folded into OpenTelemetry) and was pinned <=2.4.0, yet it was a required core dependency — pulled by every Faust install even though distributed tracing is opt-in (app.tracer defaults to None). This moves it to an optional extra, so the default install no longer depends on an unmaintained package.

This is Option A from a broader OpenTracing-removal discussion: the smallest, non-breaking step. No public API changes — TracerT, app.tracer, app.trace(), faust.sensors.distributed_tracing.TracingSensor, and faust.utils.tracing all keep working. Real tracing just now requires pip install faust[opentracing].

How it works

• New tiny no-op stand-in faust/utils/_opentracing.py (a Span/Tracer + tags/Format/follows_from/start_child_span surface, all no-ops).
• The six tracing-touching modules do a guarded import that falls back to it:

  try:
      import opentracing
  except ImportError:  # pragma: no cover
      from faust.utils import _opentracing as opentracing

  So Faust imports and runs without opentracing installed — tracing simply becomes a no-op. The only runtime path reached without a tracer is noop_span() (returns a stub span); every other opentracing reference is a type annotation or gated behind if tracer is not None.

Packaging

• Drop opentracing>=1.3.0,<=2.4.0 from requirements/requirements.txt.
• Add requirements/extras/opentracing.txt + the opentracing bundle in setup.py → faust[opentracing].
• Keep it in requirements/test.txt so CI still exercises the real opentracing path (the aiokafka tests use opentracing.Span directly).

Verification

• Without opentracing installed: import faust, create a default kafka:// App, resolve the transport registry, noop_span(), and import/instantiate TracingSensor — all work (verified by uninstalling the package).
• With opentracing installed: tests/unit/tables/test_recovery.py, test_base.py, test_aiokafka.py → 277 passed, 23 skipped.
• Lint clean (isort / black / flake8) across all changed files.

Migration note

Users who configure app.tracer or add TracingSensor should install faust[opentracing]. Worth a CHANGELOG entry; a natural follow-up (next major) is to remove the built-in tracing entirely in favor of the community opentelemetry-instrumentation-faust.

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.97%. Comparing base (50e35ad) to head (469a3cc).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #686      +/-   ##
==========================================
- Coverage   96.00%   95.97%   -0.04%     
==========================================
  Files         104      103       -1     
  Lines       11169    11069     -100     
  Branches     1206     1191      -15     
==========================================
- Hits        10723    10623     -100     
  Misses        352      352              
  Partials       94       94              

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