How to use auto-instrumentation for Uvicorn with multiple worker processes

Describe your environment

Using the following Dockerfile to simplify testing:

FROM tiangolo/uvicorn-gunicorn-fastapi:python3.8-slim

RUN pip install --no-cache-dir \
      opentelemetry-distro==0.19b0 \
      opentelemetry-instrumentation-fastapi==0.19b0

Save the file as Dockerfile, and build the Docker image it using the following command in the same folder:

docker build -t test-uvicorn .

Steps to reproduce

The Uvicorn server only has one registered endpoint at the root path. Running the server, and then accessing the endpoint (in this example, at http://localhost:5000/) will display any collected spans in the terminal.

Running the Uvicorn server with multiple workers disabled, and OpenTelemetry auto-instrumentation:

docker run --rm -p 5000:5000 test-uvicorn opentelemetry-instrument --trace-exporter console_span uvicorn main:app --host 0.0.0.0 --port 5000

Running the Uvicorn server with multiple workers enabled, and OpenTelemetry auto-instrumentation:

docker run --rm -p 5000:5000 test-uvicorn opentelemetry-instrument --trace-exporter console_span uvicorn main:app --host 0.0.0.0 --port 5000 --workers 2

What is the expected behavior?

Both modes (single-worker, and multi-worker) must correctly auto-instrument the application, and display collected spans.

What is the actual behavior?

Only the single-worker command displays collected spans. Multi-worker Uvicorn does not display any spans.

Additional context

Uvicorn uses multiprocessing to spawn workers. (https://github.com/encode/uvicorn/blob/bf1c64e2c1/uvicorn/main.py#L381-L384, and https://github.com/encode/uvicorn/blob/bf1c64e2c1/uvicorn/subprocess.py#L38). Could be related to the same issue that is blocking Gunicorn from working (#171).

Submitting the issue here, as apparently the auto-instrumentation code will be moved to this repository, based on https://github.com/open-telemetry/opentelemetry-python/issues/1532.