Extracting some JSON fields from the message

[Badger]

You could try

    grok { match => { "message" => "{(?<[@metadata][kvData]>[^}]+)}$" } }
    kv { source => "[@metadata][kvData]" field_split => "," trim_key => " " }

which will produce

          "apiType" => "HTTP",
"proxyResponseCode" => "500",
        "errorType" => "null",
 "applicationOwner" => "admin"