Add fields method (#226)

Co-authored-by: (Eliseo) Nathaniel Ruiz Nowell <enruizno@uwaterloo.ca>

sdk-extension/opentelemetry-sdk-extension-aws/CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+- Add method to return fields injected by propagator
+  ([#226](https://github.com/open-telemetry/opentelemetry-python/pull/226))
+
 ## Version 0.16b0
 
 Released 2020-11-25

sdk-extension/opentelemetry-sdk-extension-aws/src/opentelemetry/sdk/extension/aws/trace/propagation/aws_xray_format.py

@@ -274,3 +274,12 @@ class AwsXRayFormat(TextMapPropagator):
         set_in_carrier(
             carrier, TRACE_HEADER_KEY, trace_header,
         )
+
+    def fields(self):
+        """Returns a set with the fields set in `inject`.
+
+        See
+        `opentelemetry.trace.propagation.textmap.TextMapPropagator.fields`
+        """
+
+        return {TRACE_HEADER_KEY}