tests/unit/*tracing: pass current span into assertions

This allows us to avoid the flakes with OpenTelemetry's
inMemorySpanExporter that might not have received the
exported spans yet.

tests/_helpers.py

@@ -101,4 +101,7 @@ def assertSpanEvents(self, name, wantEventNames=[], span=None):
                 span = span_list[0]
 
             self.assertEqual(span.name, name)
-            self.assertEqual(len(span.events), len(wantEventNames))
+            actualEventNames = []
+            for event in span.events:
+                actualEventNames.append(event.name)
+            self.assertEqual(actualEventNames, wantEventNames)

tests/unit/test_session.py

@@ -189,7 +189,7 @@ def test_create_session_span_annotations(self):
         database.spanner_api = gax_api
         session = self._make_one(database, database_role=self.DATABASE_ROLE)
 
-        with trace_call("TestSessionSpan", session):
+        with trace_call("TestSessionSpan", session) as span:
             session.create()
 
             self.assertEqual(session.session_id, self.SESSION_ID)
@@ -209,8 +209,8 @@ def test_create_session_span_annotations(self):
                 ],
             )
 
-        wantEventNames = ["Acquiring session", "Creating Session", "Using Session"]
-        self.assertSpanEvents("CloudSpanner.CreateSession", wantEventNames)
+            wantEventNames = ["Creating Session", "Using Session"]
+            self.assertSpanEvents("TestSessionSpan", wantEventNames, span)
 
     def test_create_wo_database_role(self):
         from google.cloud.spanner_v1 import CreateSessionRequest