Exclude opt-in Element performance metrics from encryption

This is needed for the system which is meant to be monitoring this metadata. See matrix-org/matrix-react-sdk#6766

src/crypto/index.ts

@@ -2757,12 +2757,22 @@ export class Crypto extends EventEmitter {
             delete content['m.relates_to'];
         }
 
+        // Treat element's performance metrics the same as `m.relates_to` (when present)
+        const elementPerfMetrics = content['io.element.performance_metrics'];
+        if (elementPerfMetrics) {
+            content = Object.assign({}, content);
+            delete content['io.element.performance_metrics'];
+        }
+
         const encryptedContent = await alg.encryptMessage(
             room, event.getType(), content);
 
         if (mRelatesTo) {
             encryptedContent['m.relates_to'] = mRelatesTo;
         }
+        if (elementPerfMetrics) {
+            encryptedContent['io.element.performance_metrics'] = elementPerfMetrics;
+        }
 
         event.makeEncrypted(
             "m.room.encrypted",