一个tracer过程中，各span的关系


        [Span A]  ←←←(the root span)
            |
     +------+------+
     |             |
 [Span B]      [Span C] ←←←(Span C 是 Span A 的孩子节点, ChildOf)
     |             |
 [Span D]      +---+-------+
               |           |
           [Span E]    [Span F] >>> [Span G] >>> [Span H]
                                       ↑
                                       ↑
                                       ↑
                         (Span G 在 Span F 后被调用, FollowsFrom)
                         
tracer与span的时间轴关系


––|–––––––|–––––––|–––––––|–––––––|–––––––|–––––––|–––––––|–> time

 [Span A···················································]
   [Span B··············································]
      [Span D··········································]
    [Span C········································]
         [Span E·······]        [Span F··] [Span G··] [Span H··]

type Tracer interface {
    // 用于新建，启动，返回一个新的 Span
	// 比如:
	//
	//     var tracer opentracing.Tracer = ...
	//
	//     // The root-span case:
	//     sp := tracer.StartSpan("GetFeed")
	//
	//     // The vanilla child span case:
	//     sp := tracer.StartSpan(
	//         "GetFeed",
	//         opentracing.ChildOf(parentSpan.Context()))
	//
	//     // All the bells and whistles:
	//     sp := tracer.StartSpan(
	//         "GetFeed",
	//         opentracing.ChildOf(parentSpan.Context()),
	//         opentracing.Tag{"user_agent", loggedReq.UserAgent},
	//         opentracing.StartTime(loggedReq.Timestamp),
	//     )
	//
	StartSpan(operationName string, opts ...StartSpanOption) Span

	// 注入 Inject 和 Extract 是 Span 传递的关键，Inject 将 Span 信息以某种格式注入载体，
	//  Extract 则是做提取，以最常见的 Http 为例，Inject 将 Span 信息以 Http Header 的方式
	// 注入，提取的时候则 从 Http Header 提取，这里只要 Inject 和 Extract 对应就可以，你也可以
	// 定义自己的 Http 注入方式
	// 
	// 比如:
	//
	//     carrier := opentracing.HTTPHeadersCarrier(httpReq.Header)
	//     err := tracer.Inject(
	//         span.Context(),
	//         opentracing.HTTPHeaders,
	//         carrier)
	//
	Inject(sm SpanContext, format interface{}, carrier interface{}) error
    
    // 提取，这里给了一个最常见的例子，理解这个例子：StartSpan 有两种情况：一是过来的请求里面有 Span
    // 信息了，那么要 Start with clientContext；二是 过来的请求没有 Span，那么 新建一个新的无关的 Span
	// 
	// 例子：
	//
	//
	//     carrier := opentracing.HTTPHeadersCarrier(httpReq.Header)
	//     clientContext, err := tracer.Extract(opentracing.HTTPHeaders, carrier)
	//
	//     // ... assuming the ultimate goal here is to resume the trace with a
	//     // server-side Span:
	//     var serverSpan opentracing.Span
	//     if err == nil {
	//         span = tracer.StartSpan(
	//             rpcMethodName, ext.RPCServerOption(clientContext))
	//     } else {
	//         span = tracer.StartSpan(rpcMethodName)
	//     }
	Extract(format interface{}, carrier interface{}) (SpanContext, error)
}

// SpanContext represents Span state that must propagate to descendant Spans and across process
// boundaries (e.g., a <trace_id, span_id, sampled> tuple).
type SpanContext interface {
	// ForeachBaggageItem grants access to all baggage items stored in the
	// SpanContext.
	ForeachBaggageItem(handler func(k, v string) bool)
}

// Span represents an active, un-finished span in the OpenTracing system.
//
// Spans are created by the Tracer interface.
// 这里注释进行了删减，比较重要的是 SetOperationName/SetTag/LogFields => Finish
type Span interface {
	// Sets the end timestamp and finalizes Span state.
	Finish()
	// FinishWithOptions is like Finish() but with explicit control over timestamps and log data.
	FinishWithOptions(opts FinishOptions)

	// Context() yields the SpanContext for this Span. Note that the return
	// value of Context() is still valid after a call to Span.Finish(), as is
	// a call to Span.Context() after a call to Span.Finish().
	Context() SpanContext

	// Sets or changes the operation name.
	SetOperationName(operationName string) Span

	// Adds a tag to the span.
	SetTag(key string, value interface{}) Span

	// LogFields is an efficient and type-checked way to record key:value
	// logging data about a Span, though the programming interface is a little
	// more verbose than LogKV(). Here's an example:
	//
	//    span.LogFields(
	//        log.String("event", "soft error"),
	//        log.String("type", "cache timeout"),
	//        log.Int("waited.millis", 1500))
	//
	// Also see Span.FinishWithOptions() and FinishOptions.BulkLogData.
	LogFields(fields ...log.Field)

	// LogKV is a concise, readable way to record key:value logging data about
	// a Span, though unfortunately this also makes it less efficient and less
	// type-safe than LogFields(). Here's an example:
	//
	//    span.LogKV(
	//        "event", "soft error",
	//        "type", "cache timeout",
	//        "waited.millis", 1500)
	//
	LogKV(alternatingKeyValues ...interface{})

	// SetBaggageItem sets a key:value pair on this Span and its SpanContext
	// that also propagates to descendants of this Span.
	SetBaggageItem(restrictedKey, value string) Span

	// Gets the value for a baggage item given its key. Returns the empty string
	// if the value isn't found in this Span.
	BaggageItem(restrictedKey string) string

	// Provides access to the Tracer that created this Span.
	Tracer() Tracer
}

func (p Propagator) Inject(
	sc jaeger.SpanContext,
	abstractCarrier interface{},
) error {
	textMapWriter, ok := abstractCarrier.(opentracing.TextMapWriter)
	if !ok {
		return opentracing.ErrInvalidCarrier
	}

	textMapWriter.Set("x-b3-traceid", sc.TraceID().String())
	if sc.ParentID() != 0 {
		textMapWriter.Set("x-b3-parentspanid", strconv.FormatUint(uint64(sc.ParentID()), 16))
	}
	textMapWriter.Set("x-b3-spanid", strconv.FormatUint(uint64(sc.SpanID()), 16))
	if sc.IsSampled() {
		textMapWriter.Set("x-b3-sampled", "1")
	} else {
		textMapWriter.Set("x-b3-sampled", "0")
	}
	sc.ForeachBaggageItem(func(k, v string) bool {
		textMapWriter.Set(p.baggagePrefix+k, v)
		return true
	})
	return nil
}

type Message struct {
	Body string
	Extra map[string]string
}

// jaegertracing
docker run --rm -p 6831:6831/udp -p 6832:6832/udp  -p 16686:16686 jaegertracing/all-in-one:1.7 --log-level=debug
// rabbitmq, 启动后进入容器新建 root 用户，新建一个 test queue
docker run --name rabbitmq -p 15672:15672 -p 5672:5672 ccr.ccs.tencentyun.com/wajika/rabbitmq-management:3.7.8