Research if Django instrumentation can auto-detect `DJANGO_SETTINGS_MODULE` when auto-instrumentation is used

Currently users are required to set the DJANGO_SETTINGS_MODULE env var when auto-instrumenting Django. It would be convenient for the auto-instrumentation to be able to figure this out automatically.

hint: it might be possible to fix this by lazily import django.conf.settings instead of here: https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/instrumentation/opentelemetry-instrumentation-django/src/opentelemetry/instrumentation/django/__init__.py#L80

Currenly the instrumentation fails if the env var is not setup:

2021-09-10 19:05:16,525 ERROR [/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/opentelemetry/instrumentation/auto_instrumentation/sitecustomize.py] [sitecustomize.py:84] [trace_id=0 span_id=0 resource.service.name=my-django-service] - Instrumenting of django failed
Traceback (most recent call last):
  File "/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/opentelemetry/instrumentation/auto_instrumentation/sitecustomize.py", line 81, in _load_instrumentors
    distro.load_instrumentor(entry_point, skip_dep_check=True)
  File "/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/opentelemetry/instrumentation/distro.py", line 63, in load_instrumentor
    instrumentor().instrument(**kwargs)
  File "/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/opentelemetry/instrumentation/instrumentor.py", line 109, in instrument
    result = self._instrument(  # pylint: disable=assignment-from-no-return
  File "/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/opentelemetry/instrumentation/django/__init__.py", line 132, in _instrument
    settings_middleware = getattr(settings, "MIDDLEWARE", [])
  File "/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/django/conf/__init__.py", line 82, in __getattr__
    self._setup(name)
  File "/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/django/conf/__init__.py", line 63, in _setup
    raise ImproperlyConfigured(
django.core.exceptions.ImproperlyConfigured: Requested setting MIDDLEWARE, but settings are not configured. You must either define the environment variable DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.
2021-09-10 19:05:16,528 ERROR [/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/opentelemetry/instrumentation/auto_instrumentation/sitecustomize.py] [sitecustomize.py:113] [trace_id=0 span_id=0 resource.service.name=my-django-service] - Failed to auto initialize opentelemetry
Traceback (most recent call last):
  File "/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/opentelemetry/instrumentation/auto_instrumentation/sitecustomize.py", line 111, in initialize
    _load_instrumentors(distro)
  File "/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/opentelemetry/instrumentation/auto_instrumentation/sitecustomize.py", line 85, in _load_instrumentors
    raise exc
  File "/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/opentelemetry/instrumentation/auto_instrumentation/sitecustomize.py", line 81, in _load_instrumentors
    distro.load_instrumentor(entry_point, skip_dep_check=True)
  File "/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/opentelemetry/instrumentation/distro.py", line 63, in load_instrumentor
    instrumentor().instrument(**kwargs)
  File "/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/opentelemetry/instrumentation/instrumentor.py", line 109, in instrument
    result = self._instrument(  # pylint: disable=assignment-from-no-return
  File "/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/opentelemetry/instrumentation/django/__init__.py", line 132, in _instrument
    settings_middleware = getattr(settings, "MIDDLEWARE", [])
  File "/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/django/conf/__init__.py", line 82, in __getattr__
    self._setup(name)
  File "/Users/olone/playground/django-otel/venv/lib/python3.9/site-packages/django/conf/__init__.py", line 63, in _setup
    raise ImproperlyConfigured(
django.core.exceptions.ImproperlyConfigured: Requested setting MIDDLEWARE, but settings are not configured. You must either define the environment variable DJANGO_SETTINGS_MODULE or call settings.configure() before accessing settings.
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September 10, 2021 - 13:35:17
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Read instrumentation/opentelemetry-instrumentation-django/src/opentelemetry/instrumentation/django/init.py around line 80 and the _instrument path shown at line 132. Compare when django.conf.settings is imported with the auto-instrumentation flow in sitecustomize.py, then reproduce startup with and without DJANGO_SETTINGS_MODULE. Done means establishing whether settings can be detected safely and recording or implementing behavior that avoids the reported initialization failure.