Move baggage and propagation to separate packages (#1325)

* Move propagation code to propagation package

* Move baggage code to baggage package

* Update changelog

* Make docs of baggage.Set more clear

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>

propagation/propagation_test.go

@@ -0,0 +1,102 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package propagation_test
+
+import (
+	"context"
+	"strings"
+	"testing"
+
+	"go.opentelemetry.io/otel/propagation"
+)
+
+type ctxKeyType uint
+
+var (
+	ctxKey ctxKeyType = 0
+)
+
+type carrier []string
+
+func (c *carrier) Get(string) string { return "" }
+
+func (c *carrier) Set(setter, _ string) {
+	*c = append(*c, setter)
+}
+
+type propagator struct {
+	Name string
+}
+
+func (p propagator) Inject(ctx context.Context, carrier propagation.TextMapCarrier) {
+	carrier.Set(p.Name, "")
+}
+
+func (p propagator) Extract(ctx context.Context, carrier propagation.TextMapCarrier) context.Context {
+	v := ctx.Value(ctxKey)
+	if v == nil {
+		ctx = context.WithValue(ctx, ctxKey, []string{p.Name})
+	} else {
+		orig := v.([]string)
+		ctx = context.WithValue(ctx, ctxKey, append(orig, p.Name))
+	}
+	return ctx
+}
+
+func (p propagator) Fields() []string { return []string{p.Name} }
+
+func TestCompositeTextMapPropagatorFields(t *testing.T) {
+	a, b1, b2 := propagator{"a"}, propagator{"b"}, propagator{"b"}
+
+	want := map[string]struct{}{
+		"a": {},
+		"b": {},
+	}
+	got := propagation.NewCompositeTextMapPropagator(a, b1, b2).Fields()
+	if len(got) != len(want) {
+		t.Fatalf("invalid fields from composite: %v (want %v)", got, want)
+	}
+	for _, v := range got {
+		if _, ok := want[v]; !ok {
+			t.Errorf("invalid field returned from composite: %q", v)
+		}
+	}
+}
+
+func TestCompositeTextMapPropagatorInject(t *testing.T) {
+	a, b := propagator{"a"}, propagator{"b"}
+
+	c := make(carrier, 0, 2)
+	propagation.NewCompositeTextMapPropagator(a, b).Inject(context.Background(), &c)
+
+	if got := strings.Join([]string(c), ","); got != "a,b" {
+		t.Errorf("invalid inject order: %s", got)
+	}
+}
+
+func TestCompositeTextMapPropagatorExtract(t *testing.T) {
+	a, b := propagator{"a"}, propagator{"b"}
+
+	ctx := context.Background()
+	ctx = propagation.NewCompositeTextMapPropagator(a, b).Extract(ctx, nil)
+
+	v := ctx.Value(ctxKey)
+	if v == nil {
+		t.Fatal("no composite extraction")
+	}
+	if got := strings.Join(v.([]string), ","); got != "a,b" {
+		t.Errorf("invalid extract order: %s", got)
+	}
+}