How to remove dynamical field?

Elastic Stack Logstash
12

Count from the message beginning of the number of sixth .
but i don't know how to make logstash to do ?

for example,
{ "message" => "..",
"host" => "10.10.10.13",
"@version" => "1",
"@timestamp" => "2016-04-08T06:22:17.711Z",
"type" => "snmptrap",
"MSDP-MIB::msdpPeerState_10_10_11_90" => "1",
"field_7"=>"field_7 content",
"source_ip" => "10.10.10.13"
}

i want to pick the sixth field is "MSDP-MIB::msdpPeerState_10_10_11_90" => "1",
i want the output looks like as below,

"message" => "..",
"host" => "10.10.10.13",
"@version" => "1",
"@timestamp" => "2016-04-08T06:22:17.711Z",
"type" => "snmptrap",
"MSDP-MIB::msdpPeerState_10_10_11_90" => "1",
"source_ip" => "10.10.10.13"