Add baggage implementation based on the W3C and OpenTelemetry specification (#1967)

* Rename baggage context file

* Initial baggage implementation

* Initial tests

* More tests

* Update baggage context functionality

* Add New method to baggage pkg

* Update namedtracer example

* URL encode baggage values

* Refactor and use internal baggage pkg

* Update OpenTracing bridge

* Update baggage propagator

* Fix lint and test errors

* Add changes to changelog

* Apply suggestions from code review

* Rename testcase field per suggestion

* Update test to verify last-one-wins semantics

* Explicitly seed random numbers with static seed in tests

* Parse Member key/value with string split

* Add test for member parse with equal signs in value

* Trim whitespaces for member key/value

internal/baggage/context_test.go

@@ -0,0 +1,96 @@
+// 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 baggage
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestContextWithList(t *testing.T) {
+	ctx := context.Background()
+	l := List{"foo": {Value: "1"}}
+
+	nCtx := ContextWithList(ctx, l)
+	assert.Equal(t, baggageState{list: l}, nCtx.Value(baggageKey))
+	assert.Nil(t, ctx.Value(baggageKey))
+}
+
+func TestClearContextOfList(t *testing.T) {
+	l := List{"foo": {Value: "1"}}
+
+	ctx := context.Background()
+	ctx = context.WithValue(ctx, baggageKey, l)
+
+	nCtx := ContextWithList(ctx, nil)
+	nL, ok := nCtx.Value(baggageKey).(baggageState)
+	require.True(t, ok, "wrong type stored in context")
+	assert.Nil(t, nL.list)
+	assert.Equal(t, l, ctx.Value(baggageKey))
+}
+
+func TestListFromContext(t *testing.T) {
+	ctx := context.Background()
+	assert.Nil(t, ListFromContext(ctx))
+
+	l := List{"foo": {Value: "1"}}
+	ctx = context.WithValue(ctx, baggageKey, baggageState{list: l})
+	assert.Equal(t, l, ListFromContext(ctx))
+}
+
+func TestContextWithSetHook(t *testing.T) {
+	var called bool
+	f := func(ctx context.Context, list List) context.Context {
+		called = true
+		return ctx
+	}
+
+	ctx := context.Background()
+	ctx = ContextWithSetHook(ctx, f)
+	assert.False(t, called, "SetHookFunc called when setting hook")
+	ctx = ContextWithList(ctx, nil)
+	assert.True(t, called, "SetHookFunc not called when setting List")
+
+	// Ensure resetting the hook works.
+	called = false
+	ctx = ContextWithSetHook(ctx, f)
+	assert.False(t, called, "SetHookFunc called when re-setting hook")
+	ContextWithList(ctx, nil)
+	assert.True(t, called, "SetHookFunc not called when re-setting List")
+}
+
+func TestContextWithGetHook(t *testing.T) {
+	var called bool
+	f := func(ctx context.Context, list List) List {
+		called = true
+		return list
+	}
+
+	ctx := context.Background()
+	ctx = ContextWithGetHook(ctx, f)
+	assert.False(t, called, "GetHookFunc called when setting hook")
+	_ = ListFromContext(ctx)
+	assert.True(t, called, "GetHookFunc not called when getting List")
+
+	// Ensure resetting the hook works.
+	called = false
+	ctx = ContextWithGetHook(ctx, f)
+	assert.False(t, called, "GetHookFunc called when re-setting hook")
+	_ = ListFromContext(ctx)
+	assert.True(t, called, "GetHookFunc not called when re-getting List")
+}