Add envelop header

Author: cleptric

develop-docs/sdk/telemetry/spans/span-protocol.mdx

@@ -8,6 +8,26 @@ title: Span Protocol
 
 The SDK must implement a new "span v2" envelope item, which is used to emit spans to Sentry.
 
+## Span v2 Envelope Header
+
+The envelope header must contain the same properties as previously with transactions.
+There are no special requirements for the Span v2 envelope header.
+
+```json
+{
+  "sent_at": "2025-02-07T14:16:00Z",
+  "dsn": "https://[email protected]/42",
+  "sdk": {
+    // ...
+  },
+  "trace": {
+    // ...
+  }
+}
+```
+
+Also see [Envelope Headers](/sdk/data-model/envelopes/#headers).
+
 ## Span v2 Envelope Item Header
 
 The envelope item header must contain the following properties: