Add opt-in OpenTelemetry auto-instrumentation when running Python apps

Motivation

posit-dev/py-shiny#2274 is making OpenTelemetry zero-code auto-instrumentation a first-class path: shiny[otel] will include opentelemetry-distro[otlp], so opentelemetry-instrument shiny run app.py works out of the box (py-shiny resolves the global tracer provider lazily, so no app-code changes are needed).

The extension could make this a one-setting experience: run the app under auto-instrumentation so users see traces (e.g. console or OTLP to a local collector) while developing, without editing their app.

Proposal

Add an opt-in setting (e.g. shiny.python.otelInstrument) that, when enabled, wraps the Run Shiny App launch:

<env>/opentelemetry-instrument <python> -m shiny run --port ... --reload ... app.py

Implementation notes (verified experimentally):

• The opentelemetry-instrument entry-point script must be resolved next to the selected interpreter (dirname(python)/opentelemetry-instrument, or Scripts\opentelemetry-instrument.exe on Windows). python -m opentelemetry.instrumentation.auto_instrumentation does not work — the package has no __main__.
• Wrapping <python> -m shiny run this way works: Shiny's session_start/reactive_update/session_end spans export correctly, and --reload is compatible (the reloaded process inherits instrumentation).
• Exporter/config selection can be left to standard OTEL_* env vars / flags; a companion setting for extra args (e.g. --traces_exporter console) may be handy for a batteries-included default.
• If the script isn't found in the environment, show a friendly message suggesting pip install 'shiny[otel]'.
• Scope: Run only for a first pass. The Debug launch uses vscode.debug.startDebugging with module: shiny, which doesn't compose trivially with the wrapper — can be a follow-up.

References

• Issue that motivated this: posit-dev/py-shiny#2274
• Docs PR: posit-dev/py-shiny-site#380

Start at the Run Shiny App launch path and inspect how it selects the Python interpreter and builds the python -m shiny run command. Verify the wrapper using opentelemetry-instrument shiny run app.py, including the interpreter-adjacent script path and the missing-script case. Done means an opt-in setting wraps Run launches, preserves reload behavior, and gives a friendly install hint; Debug is out of scope.