Tracing: traces now correctly form a tree (#1170)

* First cut of trace

* Dial it back yo

* Defensive programming

debug/trace/trace.go

@@ -3,6 +3,7 @@ package trace
 
 import (
 	"context"
+	"github.com/micro/go-micro/v2/metadata"
 	"time"
 )
 
@@ -34,17 +35,32 @@ type Span struct {
 	Metadata map[string]string
 }
 
-type spanKey struct{}
+const (
+	traceIDKey = "Micro-Trace-Id"
+	spanIDKey  = "Micro-Span-Id"
+)
 
 // FromContext returns a span from context
-func FromContext(ctx context.Context) (*Span, bool) {
-	s, ok := ctx.Value(spanKey{}).(*Span)
-	return s, ok
+func FromContext(ctx context.Context) (traceID string, parentSpanID string, isFound bool) {
+	traceID, traceOk := metadata.Get(ctx, traceIDKey)
+	microID, microOk := metadata.Get(ctx, "Micro-Id")
+	if !traceOk && !microOk {
+		isFound = false
+		return
+	}
+	if !traceOk {
+		traceID = microID
+	}
+	parentSpanID, ok := metadata.Get(ctx, spanIDKey)
+	return traceID, parentSpanID, ok
 }
 
-// NewContext creates a new context with the span
-func NewContext(ctx context.Context, s *Span) context.Context {
-	return context.WithValue(ctx, spanKey{}, s)
+// ToContext saves the trace and span ids in the context
+func ToContext(ctx context.Context, traceID, parentSpanID string) context.Context {
+	return metadata.MergeContext(ctx, map[string]string{
+		traceIDKey: traceID,
+		spanIDKey:  parentSpanID,
+	}, true)
 }
 
 var (