chore: Add user agent to .sdk_metadata.json (#224)

Author: kinyoklion

Diff for: .sdk_metadata.json

@@ -6,7 +6,8 @@
       "type": "server-side",
       "languages": [
         "PHP"
-      ]
+      ],
+      "userAgents": ["PHPClient"]
     }
   }
 }