Make NullContextManager support nested spans (#156)

Author: liyanhui1228

opencensus/trace/tracers/base.py

@@ -72,7 +72,10 @@ def __init__(self, span_id=None):
         self.span_id = span_id
 
     def __enter__(self):
-        pass  # pragma: NO COVER
+        return self  # pragma: NO COVER
 
     def __exit__(self, exc_type, exc_value, traceback):
         pass  # pragma: NO COVER
+
+    def span(self, name='span'):
+        return NullContextManager()

tests/unit/trace/test_tracer.py

@@ -141,7 +141,12 @@ def test_span_not_sampled(self):
         tracer = tracer_module.Tracer(sampler=sampler)
 
         span = tracer.span()
+
+        # Test nested span not sampled
+        child_span = span.span()
+
         assert isinstance(span, base.NullContextManager)
+        assert isinstance(child_span, base.NullContextManager)
 
     def test_span_sampled(self):
         sampler = mock.Mock()