SDK: set sampled flag on sampling trace (#407)

Author: mauriciovasquezbernal

Diff for: opentelemetry-sdk/src/opentelemetry/sdk/trace/__init__.py

@@ -509,6 +509,8 @@ def start_span(  # pylint: disable=too-many-locals
         )
 
         if sampling_decision.sampled:
+            options = context.trace_options | trace_api.TraceOptions.SAMPLED
+            context.trace_options = trace_api.TraceOptions(options)
             if attributes is None:
                 span_attributes = sampling_decision.attributes
             else:

Diff for: opentelemetry-sdk/tests/trace/test_trace.py

@@ -117,6 +117,7 @@ def test_default_sampler(self):
         self.assertIsInstance(root_span, trace.Span)
         child_span = tracer.start_span(name="child span", parent=root_span)
         self.assertIsInstance(child_span, trace.Span)
+        self.assertTrue(root_span.context.trace_options.sampled)
 
     def test_sampler_no_sampling(self):
         tracer_source = trace.TracerSource(sampling.ALWAYS_OFF)
@@ -251,6 +252,9 @@ def test_start_span_explicit(self):
         other_parent = trace_api.SpanContext(
             trace_id=0x000000000000000000000000DEADBEEF,
             span_id=0x00000000DEADBEF0,
+            trace_options=trace_api.TraceOptions(
+                trace_api.TraceOptions.SAMPLED
+            ),
         )
 
         self.assertIsNone(tracer.get_current_span())