fix: defer boot_agent until after eventlet monkey_patch to prevent recursion error

[CagriYonca]

Problem & Root Cause

When running under Gunicorn with the eventlet worker class, Instana's early auto-instrumentation eagerly imports third-party libraries in the master process before eventlet.monkey_patch executes.

Modules such as urllib3 and pymongo capture module-level references to the unpatched ssl.SSLContext. When Eventlet subsequently patches the ssl module, ssl.SSLContext is replaced with its green subclass. Initializing SSL connections later causes super lookups inside property setters to resolve the global green subclass instead of the original class, resulting in an infinite recursion and a RecursionError.

Solution

This PR defers boot_agent execution when eventlet is present in the environment:

• Wraps eventlet.monkey_patch using wrapt.
• Defers boot_agent and all eager instrumentation imports until after eventlet.monkey_patch completes in the forked worker process.
• Suppresses sensor boot in the Gunicorn arbiter process, ensuring instrumentation runs only in worker processes.

Changes

• src/instana/__init__.py: Added _defer_boot_until_eventlet_patch to defer agent boot until worker post-patch.
• tests/frameworks/test_eventlet_autotrace.py: Added unit tests covering deferred boot registration, worker execution, and arbiter suppression.

[pvital]

Great work @CagriYonca.
I have a few considerations.

[GSVarsha]

Few requests.

[CagriYonca]

Update after comments:

• Prevent double boot_agent() execution: Handled module-level unconditional call with a return_early flag and added an _eventlet_booted guard per process.

• Fix Gunicorn 26+ crash on startup: Replaced eager wrap_function_wrapper("gunicorn.workers.geventlet", ...) with wrapt.when_imported(...) to prevent ModuleNotFoundError when the removed eventlet worker is not present.

• Remove fragile /proc inspection: Replaced /proc/{ppid}/cmdline checking (which failed under Docker PID 1, console scripts, and python -m gunicorn) by hooking directly into EventletWorker.patch(), running exclusively post-fork in workers.

• Make deferred boot opt-in: Restricted deferred initialization behind INSTANA_EVENTLET_DEFERRED_BOOT=true so non-eventlet environments (Celery, gthread, standalone scripts) don't silently lose tracing.

• Fix SQLAlchemy test collection: Moved module-level create_engine(...) into a fixture and explicitly pinned the dialect to postgresql+psycopg2:// to avoid Pytest collection failures on Python 3.11+ / SQLAlchemy 2.1+.