Fix the empty headers use case for OTTracePropagator (#378)

Author: marcinzaremba

Diff for: CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased](https://github.com/open-telemetry/opentelemetry-python-contrib/compare/v0.18b0...HEAD)
 - Updated instrumentations to use `opentelemetry.trace.use_span` instead of `Tracer.use_span()`
   ([#364](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/364))
+- `opentelemetry-propagator-ot-trace` Do not throw an exception when headers are not present
+  ([#378](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/378))
 
 ### Changed
 - Rename `IdsGenerator` to `IdGenerator`

Diff for: propagator/opentelemetry-propagator-ot-trace/src/opentelemetry/propagators/ot_trace/__init__.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 from re import compile as re_compile
-from typing import Iterable, Optional
+from typing import Any, Iterable, Optional
 
 from opentelemetry.baggage import get_all, set_baggage
 from opentelemetry.context import Context
@@ -55,10 +55,12 @@ def extract(
     ) -> Context:
 
         traceid = _extract_first_element(
-            getter.get(carrier, OT_TRACE_ID_HEADER)
+            getter.get(carrier, OT_TRACE_ID_HEADER), INVALID_TRACE_ID
         )
 
-        spanid = _extract_first_element(getter.get(carrier, OT_SPAN_ID_HEADER))
+        spanid = _extract_first_element(
+            getter.get(carrier, OT_SPAN_ID_HEADER), INVALID_SPAN_ID
+        )
 
         sampled = _extract_first_element(
             getter.get(carrier, OT_SAMPLED_HEADER)
@@ -163,8 +165,8 @@ def fields(self):
 
 
 def _extract_first_element(
-    items: Iterable[TextMapPropagatorT],
+    items: Iterable[TextMapPropagatorT], default: Any = None,
 ) -> Optional[TextMapPropagatorT]:
     if items is None:
-        return None
+        return default
     return next(iter(items), None)

Diff for: propagator/opentelemetry-propagator-ot-trace/tests/test_ot_trace_propagator.py

@@ -366,3 +366,12 @@ def test_extract_baggage(self):
 
         self.assertEqual(baggage["abc"], "abc")
         self.assertEqual(baggage["def"], "def")
+
+    def test_extract_empty(self):
+        "Test extraction when no headers are present"
+
+        span_context = get_current_span(
+            self.ot_trace_propagator.extract(carrier_getter, {})
+        ).get_span_context()
+
+        self.assertEqual(span_context, INVALID_SPAN_CONTEXT)