feat: Add span links to Span V2 (#4737)

The link schema for Span V2 has been finalized. This implements it
according to the Notion doc, which means that it's the same as the
schema for V1 span links, except for the `attributes` collection
containing typed values.

Closes RELAY-71.

#skip-changelog

Author: loewenheim

relay-event-schema/src/protocol/span_v2.rs

@@ -1,4 +1,4 @@
-use relay_protocol::{Annotated, Empty, FromValue, IntoValue, Object, Value};
+use relay_protocol::{Annotated, Array, Empty, FromValue, IntoValue, Object, Value};
 
 use std::fmt;
 
@@ -57,6 +57,10 @@ pub struct SpanV2 {
     #[metastructure(required = true)]
     pub end_timestamp: Annotated<Timestamp>,
 
+    /// Links from this span to other spans.
+    #[metastructure(pii = "maybe")]
+    pub links: Annotated<Array<SpanV2Link>>,
+
     /// Arbitrary attributes on a span.
     #[metastructure(pii = "true", trim = false)]
     pub attributes: Annotated<Object<Attribute>>,
@@ -255,6 +259,31 @@ impl IntoValue for SpanV2Kind {
     }
 }
 
+/// A link from a span to another span.
+#[derive(Clone, Debug, Default, PartialEq, Empty, FromValue, IntoValue, ProcessValue)]
+#[metastructure(trim = false)]
+pub struct SpanV2Link {
+    /// The trace id of the linked span.
+    #[metastructure(required = true, trim = false)]
+    pub trace_id: Annotated<TraceId>,
+
+    /// The span id of the linked span.
+    #[metastructure(required = true, trim = false)]
+    pub span_id: Annotated<SpanId>,
+
+    /// Whether the linked span was positively/negatively sampled.
+    #[metastructure(trim = false)]
+    pub sampled: Annotated<bool>,
+
+    /// Span link attributes, similar to span attributes/data.
+    #[metastructure(pii = "maybe", trim = false)]
+    pub attributes: Annotated<Object<Attribute>>,
+
+    /// Additional arbitrary fields for forwards compatibility.
+    #[metastructure(additional_properties, pii = "maybe", trim = false)]
+    pub other: Object<Value>,
+}
+
 #[cfg(test)]
 mod tests {
     use chrono::{TimeZone, Utc};
@@ -287,6 +316,19 @@ mod tests {
   "kind": "server",
   "start_timestamp": 1742921669.25,
   "end_timestamp": 1742921669.75,
+  "links": [
+    {
+      "trace_id": "627a2885119dcc8184fae7eef09438cb",
+      "span_id": "6c71fc6b09b8b716",
+      "sampled": true,
+      "attributes": {
+        "sentry.link.type": {
+          "type": "string",
+          "value": "previous_trace"
+        }
+      }
+    }
+  ],
   "attributes": {
     "custom.error_rate": {
       "type": "double",
@@ -348,6 +390,16 @@ mod tests {
             "server.address" => "DHWKN7KX6N.local", String,
             "http.response.status_code" => 200i64, Integer,
         );
+
+        let links = vec![Annotated::new(SpanV2Link {
+            trace_id: Annotated::new("627a2885119dcc8184fae7eef09438cb".parse().unwrap()),
+            span_id: Annotated::new(SpanId("6c71fc6b09b8b716".into())),
+            sampled: Annotated::new(true),
+            attributes: Annotated::new(attrs!(
+                "sentry.link.type" => "previous_trace", String
+            )),
+            ..Default::default()
+        })];
         let span = Annotated::new(SpanV2 {
             start_timestamp: Annotated::new(
                 Utc.timestamp_opt(1742921669, 250000000).unwrap().into(),
@@ -360,6 +412,7 @@ mod tests {
             status: Annotated::new(SpanV2Status::Ok),
             kind: Annotated::new(SpanV2Kind::Server),
             is_remote: Annotated::new(true),
+            links: Annotated::new(links),
             attributes: Annotated::new(attributes),
             ..Default::default()
         });