Here's an example that uses the multiline codec (prefered over the multiline filter, as it's more appropriate when you might have more than one input)

input {
  file {
    path => "/opt/IBM/tivoli/common/CTGIM/logs/access.log"
    type => "itim-access"
    start_position => "beginning"
    sincedb_path => "/dev/null"
    codec => multiline {
      pattern => "^<Message Id"
      negate => true
      what => previous

Getting a match can be difficult, as grok by default does not match against multiple lines. You can mutate to remvove all the new lines, or use a seeming secret preface, the '(?m)' directive as shown below

filter {
  grok {
    match => { "message" => "(?m)(?<timestamp>%{YEAR}.%{MONTHNUM}.%{MONTHDAY} %{HOUR}:%{MINUTE}:%{SECOND}%{ISO8601_TIMEZONE})%{DATA}{WORD:catagory}%{DATA}CDATA\[%{DATA:auth}\]%{DATA}CDATA\[%{DATA:clientip}\]"}