Add fields method (#226)

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

exporter/opentelemetry-exporter-datadog/CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+- Add fields method
+  ([#226](https://github.com/open-telemetry/opentelemetry-python/pull/226))
+
 ## Version 0.15b0
 
 Released 2020-11-02

exporter/opentelemetry-exporter-datadog/src/opentelemetry/exporter/datadog/propagator.py

@@ -106,6 +106,19 @@ class DatadogFormat(TextMapPropagator):
                 span.context.trace_state[constants.DD_ORIGIN],
             )
 
+    def fields(self):
+        """Returns a set with the fields set in `inject`.
+
+        See
+        `opentelemetry.trace.propagation.textmap.TextMapPropagator.fields`
+        """
+        return {
+            self.TRACE_ID_KEY,
+            self.PARENT_ID_KEY,
+            self.SAMPLING_PRIORITY_KEY,
+            self.ORIGIN_KEY,
+        }
+
 
 def format_trace_id(trace_id: int) -> str:
     """Format the trace id for Datadog."""