Use a weak reference to sqlalchemy Engine to avoid memory leak (#1771)

* Use a weak reference to sqlalchemy Engine to avoid memory leak

Closes #1761

By using a weak reference to the `Engine` object, we can avoid the memory leak as disposed `Engines` get properly deallocated. Whenever `SQLAlchemy` is uninstrumented, we only trigger a removal for those event listeners which are listening for objects that haven't been garbage-collected yet.

* Made a mistake in resolving the weak reference

* Fixed formatting issues

* Updated changelog

* Added unit test to check that engine was garbage collected

* Do not save engine in EngineTracer to avoid memory leak

* Add an empty line to satisfy black formatter

* Fix isort complaints

* Fixed the issue when pool name is not set and =None

* Fix formatting issue

* Rebased after changes in a recent commit

* Updated PR number in changelog

---------

Co-authored-by: Shalev Roda <65566801+shalevr@users.noreply.github.com>

instrumentation/opentelemetry-instrumentation-sqlalchemy/tests/test_sqlalchemy.py

@@ -307,3 +307,26 @@ class TestSqlalchemyInstrumentation(TestBase):
         cnx.execute("SELECT 1 + 1;").fetchall()
         spans = self.memory_exporter.get_finished_spans()
         self.assertEqual(len(spans), 0)
+
+    def test_no_memory_leakage_if_engine_diposed(self):
+        SQLAlchemyInstrumentor().instrument()
+        import gc
+        import weakref
+
+        from sqlalchemy import create_engine
+
+        callback = mock.Mock()
+
+        def make_shortlived_engine():
+            engine = create_engine("sqlite:///:memory:")
+            # Callback will be called if engine is deallocated during garbage
+            # collection
+            weakref.finalize(engine, callback)
+            with engine.connect() as conn:
+                conn.execute("SELECT 1 + 1;").fetchall()
+
+        for _ in range(0, 5):
+            make_shortlived_engine()
+
+        gc.collect()
+        assert callback.call_count == 5