Support imudp worker thread statistics

ref: https://www.rsyslog.com/rsyslog-statistic-counter-plugin-imudp/

Basic status line:

    { "name": "imudp(w0)",
      "origin": "imudp",
      "called.recvmmsg": 1252400,
      "called.recvmsg": 0,
      "msgs.received": 619983
    }

Response:

rsyslog_input_called_recvmmsg{worker="imudp(w0)"} 77
rsyslog_input_called_recvmmsg{worker="imudp(w1)"} 84
rsyslog_input_called_recvmsg{worker="imudp(w0)"} 0
rsyslog_input_called_recvmsg{worker="imudp(w1)"} 0
rsyslog_input_received{worker="imudp(w0)"} 53
rsyslog_input_received{worker="imudp(w1)"} 48