serialize span links in meta before finishing span

ddtrace/tracer/span.go

@@ -10,6 +10,7 @@ package tracer
 import (
 	"context"
 	"encoding/base64"
+	"encoding/json"
 	"fmt"
 	"os"
 	"reflect"
@@ -469,6 +470,21 @@ func (s *span) AddSpanLinks(spanLinks ...ddtrace.SpanLink) {
 	s.SpanLinks = append(s.SpanLinks, spanLinks...)
 }
 
+// serializeSpanLinksInMeta saves span links as a JSON string under `Span[meta][_dd.span_links]`.
+func (s *span) serializeSpanLinksInMeta() {
+	if len(s.SpanLinks) == 0 {
+		return
+	}
+	spanLinkBytes, err := json.Marshal(s.SpanLinks)
+	if err != nil {
+		return
+	}
+	if s.Meta == nil {
+		s.Meta = make(map[string]string)
+	}
+	s.Meta["_dd.span_links"] = string(spanLinkBytes)
+}
+
 // Finish closes this Span (but not its children) providing the duration
 // of its part of the tracing session.
 func (s *span) Finish(opts ...ddtrace.FinishOption) {
@@ -519,6 +535,8 @@ func (s *span) Finish(opts ...ddtrace.FinishOption) {
 		}
 	}
 
+	s.serializeSpanLinksInMeta()
+
 	s.finish(t)
 	orchestrion.GLSPopValue(sharedinternal.ActiveSpanKey)
 }