chore: [SVLS-5265] add span links to encoded spans (#10850)

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

ddtrace/internal/encoding.py

@@ -83,6 +83,9 @@ def _span_to_dict(span):
         if span.span_type:
             d["type"] = span.span_type
 
+        if span._links:
+            d["span_links"] = [link.to_dict() for link in span._links]
+
         return d
 
 

tests/integration/test_encoding.py

@@ -63,3 +63,22 @@ def test_trace_with_metrics_accepted_by_agent(self, metrics):
             tracer.shutdown()
         log.warning.assert_not_called()
         log.error.assert_not_called()
+
+    @pytest.mark.parametrize(
+        "span_links_kwargs",
+        [
+            {"trace_id": 12345, "span_id": 67890},
+        ],
+    )
+    def test_trace_with_links_accepted_by_agent(self, span_links_kwargs):
+        """Links should not break things."""
+        tracer = Tracer()
+        with mock.patch("ddtrace.internal.writer.writer.log") as log:
+            with tracer.trace("root", service="test_encoding", resource="test_resource") as root:
+                root.set_link(**span_links_kwargs)
+                for _ in range(10):
+                    with tracer.trace("child") as child:
+                        child.set_link(**span_links_kwargs)
+            tracer.shutdown()
+        log.warning.assert_not_called()
+        log.error.assert_not_called()

tests/tracer/test_encoders.py

@@ -162,7 +162,7 @@ def test_encode_traces_json(self):
         # test encoding for JSON format
         traces = [
             [
-                Span(name="client.testing"),
+                Span(name="client.testing", links=[SpanLink(trace_id=12345, span_id=678990)]),
                 Span(name="client.testing"),
             ],
             [
@@ -184,6 +184,7 @@ def test_encode_traces_json(self):
         assert isinstance(spans, str)
         assert len(items) == 3
         assert len(items[0]) == 2
+        assert len(items[0][0]["span_links"]) == 1
         assert len(items[1]) == 2
         assert len(items[2]) == 2
         for i in range(3):
@@ -194,7 +195,7 @@ def test_encode_traces_json_v2(self):
         # test encoding for JSON format
         traces = [
             [
-                Span(name="client.testing", span_id=0xAAAAAA),
+                Span(name="client.testing", span_id=0xAAAAAA, links=[SpanLink(trace_id=12345, span_id=67890)]),
                 Span(name="client.testing", span_id=0xAAAAAA),
             ],
             [
@@ -215,6 +216,7 @@ def test_encode_traces_json_v2(self):
         assert isinstance(spans, str)
         assert len(items) == 3
         assert len(items[0]) == 2
+        assert len(items[0][0]["span_links"]) == 1
         assert len(items[1]) == 2
         assert len(items[2]) == 2
         for i in range(3):