a ruby-script working as a check_mk-plugin to monitor zookeeper, place it in /usr/lib/check_mk_agent/local

sample-output

$ ./zkmon.rb 
0 zk_version  - OK 3.4.3-1240972, built on 02/06/2012 10:48 GMT
0 zk_avg_latency count=5 OK 5
0 zk_max_latency count=265 OK 265
0 zk_min_latency count=0 OK 0
0 zk_packets_received count=2158687 OK 2158687
0 zk_packets_sent count=2319327 OK 2319327
0 zk_outstanding_requests count=0 OK 0
0 zk_server_state  - OK standalone
0 zk_znode_count count=10078 OK 10078
0 zk_watch_count count=0 OK 0
0 zk_ephemerals_count count=18 OK 18
0 zk_approximate_data_size count=10268580 OK 10268580
0 zk_open_file_descriptor_count count=53 OK 53
0 zk_max_file_descriptor_count count=1024 OK 1024
$

zkmon.rb

#!/usr/bin/env ruby

server = "localhost"
port = "2181"

fields=["zk_server_state","zk_version", "zk_avg_latency", "zk_max_latency", "zk_min_latency", "zk_packets_received", "zk_packets_sent", "zk_outstanding_requests", "zk_znode_count", "zk_watch_count", "zk_ephemerals_count", "zk_approximate_data_size", "zk_open_file_descriptor_count", "zk_max_file_descriptor_count"]

# First: Health check
health=`echo "ruok" | nc #{server} 2181`
if !health.match(/imok/)
  fields.each {|key|
    print "2 #{key} - CRITICAL not imok\n"
  }
  exit
end 

# max percentage for zk_open_file_descriptor_count
mpzk_open_file=0.6

mon=`echo "mntr" | nc #{server} 2181`
mhash=Hash[*mon.split(/\n/).collect { |line| 
  [line.split(/\t/)[0], line.split(/\t/,2)[1]]
}.flatten]

# first try to get Criticals (including count)
fdcount=mhash["zk_open_file_descriptor_count"].to_i
max_fdcount=mhash["zk_max_file_descriptor_count"].to_i

mhash["zk_open_file_descriptor_count"] = fdcount > max_fdcount * mpzk_open_file ? "count="+ fdcount.to_s + " CRITICAL " + fdcount.to_s : "count="+ fdcount.to_s + " OK " + fdcount.to_s 
fields.delete("zk_open_file_descriptor_count")

# keys not returning numbers
["zk_server_state","zk_version"].each {|key|
  mhash[key]=" - OK "+mhash[key].to_s
}
fields.delete("zk_server_state")
fields.delete("zk_version")

# insert count= to the rest
fields.each {|thekey|
  mhash[thekey]="count=" + mhash[thekey].to_s + " OK " + mhash[thekey].to_s
}

# now let's dump it
mhash.each_key do |key|
  if mhash[key].match(" OK ")
    rtvalue = 0
  end
  if mhash[key].match(" WARNING ")
    rtvalue = 1
  end
  if mhash[key].match(" CRITICAL ")
    rtvalue = 2
  end
  print "#{rtvalue} #{key} #{mhash[key]}\n"
end

Unfortunately, ruby is not an appropriate language to base check_mk plugins upon. The check needs to be really on a good performance in order to leverage the performance-advantages of check_mk.