feat: support baggage propagation in httpc (#2375)

* feat: support baggage propagation in httpc

* chore: use go 1.16

* chore: use go 1.16

* chore: use go ^1.16

* chore: remove deprecated

core/trace/propagation.go

@@ -0,0 +1,11 @@
+package trace
+
+import (
+	"go.opentelemetry.io/otel"
+	"go.opentelemetry.io/otel/propagation"
+)
+
+func init() {
+	otel.SetTextMapPropagator(propagation.NewCompositeTextMapPropagator(
+		propagation.TraceContext{}, propagation.Baggage{}))
+}