Add optional faust[opentelemetry] extra + docs for OpenTelemetry via the OpenTracing shim

[wbarnha]

Description

A small, non-breaking step toward OpenTelemetry: package + document the bridge that already works.

Faust's app.tracer accepts any OpenTracing-compatible tracer, and OpenTelemetry ships an official OpenTracing shim (opentelemetry.shim.opentracing_shim.create_tracer) that exposes exactly that, backed by an OTel TracerProvider. So OpenTelemetry works today with no change to Faust's tracing internals or public API — this PR just makes it a first-class, discoverable path.

Changes

• requirements/extras/opentelemetry.txt + the opentelemetry bundle in setup.py → pip install "faust[opentelemetry]" (pulls opentelemetry-opentracing-shim + opentelemetry-sdk).
• docs/userguide/sensors.rst — a new Distributed Tracing section showing how to build an app.tracer from an OpenTelemetry TracerProvider via create_tracer(), so Faust's stream/agent/rebalance spans flow out to any OTel exporter (OTLP/Jaeger/Zipkin).

Verification

• Installed the extra and ran the exact docs example: wiring app.tracer = OpenTelemetryTracer(provider) and exercising it the way Faust does internally (get_tracer(...).start_span(operation_name=…, tags=…), set_tag, finish, and app.tracer.trace(...)) — all work end-to-end, emitting OTel spans (TracerShim/SpanShim).
• setup.py lint clean (flake8/black); the RST parses cleanly and uses the file's existing .. sourcecode:: convention.

Context

This is step 2 of a phased OpenTracing→OpenTelemetry plan: #686 makes opentracing optional; this documents/packages the OTel bridge (no breaking change); a future major could do a native OTel rewrite or drop built-in tracing in favor of opentelemetry-instrumentation-faust. It complements #681 (which documents the external auto-instrumentation package) by covering the app.tracer wiring path.

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.14%. Comparing base (f3019aa) to head (1387a6e).

Additional details and impacted files

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##           master     #688   +/-   ##
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  Coverage   94.14%   94.14%           
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  Files         104      104           
  Lines       11136    11136           
  Branches     1201     1201           
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  Hits        10484    10484           
  Misses        551      551           
  Partials      101      101           

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