chore: add fields operation to TextMapPropagator (open-telemetry#1615)

Author: dyladan

api/src/context/propagation/NoopTextMapPropagator.ts

@@ -27,6 +27,9 @@ export class NoopTextMapPropagator implements TextMapPropagator {
   extract(context: Context, carrier: unknown): Context {
     return context;
   }
+  fields(): string[] {
+    return [];
+  }
 }
 
 export const NOOP_TEXT_MAP_PROPAGATOR = new NoopTextMapPropagator();

api/src/context/propagation/TextMapPropagator.ts

@@ -64,6 +64,14 @@ export interface TextMapPropagator<Carrier = any> {
     carrier: Carrier,
     getter: TextMapGetter<Carrier>
   ): Context;
+
+  /**
+   * Return a list of all fields which may be used by the propagator.
+   *
+   * This list should be used to clear fields before calling inject if a carrier is
+   * used more than once.
+   */
+  fields(): string[];
 }
 
 /**