GENERATOR_STATS is not thread-safe

[petersalas]

Describe the bug
GENERATOR_STATS is a singleton (with global class attributes) but is mutated freely. If the schema (with errors or warnings) is generated concurrently (e.g. by SpectacularAPIView) exceptions and/or invalid traces can occur.

To Reproduce
Create a schema with at least one warning or error, add an API view and then run

while true; do
    if ! curl http://localhost:8000/api/schema/ -s --fail > /dev/null; then
        break
    fi
done

in multiple bash shells against the schema URL in parallel -- eventually requests will fail.

Expected behavior
API views and schema generation are thread-safe.