add ability to append/extract tracing context

support/trace/context.go

@@ -0,0 +1,14 @@
+package trace
+
+import "context"
+
+type key struct{}
+var id = key{}
+
+func AppendTracingContext(goCtx context.Context, tracingContext interface{})  context.Context {
+	return context.WithValue(goCtx, id, tracingContext)
+}
+
+func ExtractTracingContext(goCtx context.Context) interface{} {
+	return goCtx.Value(id)
+}