Add WithSpan (#153)

Author: rainbend

README.md

@@ -110,6 +110,14 @@ Get the `activeSpan` in the `Context`.
 go2sky.ActiveSpan(ctx)
 ```
 
+## With Span
+
+Save the `activeSpan` to `Context`
+
+```go
+go2sky.WithSpan(ctx, activeSpan)
+```
+
 ## Get Global Service Name
 
 Get the `ServiceName` of the `activeSpan` in the `Context`.

trace_context.go

@@ -81,6 +81,10 @@ func ActiveSpan(ctx context.Context) Span {
 	return nil
 }
 
+func WithSpan(ctx context.Context, span Span) context.Context {
+	return context.WithValue(ctx, ctxKeyInstance, span)
+}
+
 func extractSpanString(ctx context.Context, noopResult string) (*segmentSpan, string, bool) {
 	activeSpan := ctx.Value(ctxKeyInstance)
 	if activeSpan != nil {