swarm: integrate OpenTracing; propagate ctx to internal APIs (#17169)

* swarm: propagate ctx, enable opentracing

* swarm/tracing: log error when tracing is misconfigured

vendor/github.com/uber/jaeger-client-go/contrib_observer.go

@@ -0,0 +1,56 @@
+// Copyright (c) 2017 Uber Technologies, Inc.
+//
+// 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 jaeger
+
+import (
+	opentracing "github.com/opentracing/opentracing-go"
+)
+
+// ContribObserver can be registered with the Tracer to receive notifications
+// about new Spans. Modelled after github.com/opentracing-contrib/go-observer.
+type ContribObserver interface {
+	// Create and return a span observer. Called when a span starts.
+	// If the Observer is not interested in the given span, it must return (nil, false).
+	// E.g :
+	//     func StartSpan(opName string, opts ...opentracing.StartSpanOption) {
+	//         var sp opentracing.Span
+	//         sso := opentracing.StartSpanOptions{}
+	//         if spanObserver, ok := Observer.OnStartSpan(span, opName, sso); ok {
+	//             // we have a valid SpanObserver
+	//         }
+	//         ...
+	//     }
+	OnStartSpan(sp opentracing.Span, operationName string, options opentracing.StartSpanOptions) (ContribSpanObserver, bool)
+}
+
+// ContribSpanObserver is created by the Observer and receives notifications
+// about other Span events. This interface is meant to match
+// github.com/opentracing-contrib/go-observer, via duck typing, without
+// directly importing the go-observer package.
+type ContribSpanObserver interface {
+	OnSetOperationName(operationName string)
+	OnSetTag(key string, value interface{})
+	OnFinish(options opentracing.FinishOptions)
+}
+
+// wrapper observer for the old observers (see observer.go)
+type oldObserver struct {
+	obs Observer
+}
+
+func (o *oldObserver) OnStartSpan(sp opentracing.Span, operationName string, options opentracing.StartSpanOptions) (ContribSpanObserver, bool) {
+	spanObserver := o.obs.OnStartSpan(operationName, options)
+	return spanObserver, spanObserver != nil
+}