Provide annotation to control opentracing

By default you might want opentracing off, but on for a particular
ingress.

Similarly, you might want opentracing globally on, but disabled for
a specific endpoint. To achieve this, `opentracing_propagate_context`
cannot be set when combined with `opentracing off`

A new annotation, `enable-opentracing` allows more fine grained control
of opentracing for specific ingresses.

internal/ingress/annotations/opentracing/main.go

@@ -0,0 +1,57 @@
+/*
+Copyright 2019 The Kubernetes 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 opentracing
+
+import (
+	networking "k8s.io/api/networking/v1beta1"
+
+	"k8s.io/ingress-nginx/internal/ingress/annotations/parser"
+	"k8s.io/ingress-nginx/internal/ingress/resolver"
+)
+
+type opentracing struct {
+	r resolver.Resolver
+}
+
+// Config contains the configuration to be used in the Ingress
+type Config struct {
+	Enabled bool `json:"enabled"`
+	Set     bool `json:"set"`
+}
+
+// Equal tests for equality between two Config types
+func (bd1 *Config) Equal(bd2 *Config) bool {
+	if bd1.Set != bd2.Set {
+		return false
+	} else if bd1.Enabled != bd2.Enabled {
+		return false
+	}
+	return true
+}
+
+// NewParser creates a new serviceUpstream annotation parser
+func NewParser(r resolver.Resolver) parser.IngressAnnotation {
+	return opentracing{r}
+}
+
+func (s opentracing) Parse(ing *networking.Ingress) (interface{}, error) {
+	enabled, err := parser.GetBoolAnnotation("enable-opentracing", ing)
+	if err != nil {
+		return &Config{Set: false, Enabled: false}, nil
+	}
+	return &Config{Set: true, Enabled: enabled}, nil
+}