add queue length

getJenkinsUsage.groovy

def busyExecutors = Jenkins.instance.computers
  .collect { 
    c -> c.executors.findAll { it.isBusy() }
  }
  .flatten() // reminder: transforms list(list(executor)) into list(executor)
active  = 0
busy    = 0
offline = 0
agents  = 0

busyExecutors.each { e -> 
  active = active + 1
}
Jenkins.instance.computers.each{
    if(it.isOffline()) {
      offline = offline + 1
    } else {
      if(!it.isIdle()) {
        busy = busy + 1
      }
      agents = agents + 1
    }
}

println("Num builds in queue: "+ Jenkins.instance.queue.items.length)
println("Num active jobs    : "+ active)
println("Num busy agents    : "+ busy)
println("Num offline        : "+ offline)
println("Num agents         : "+ agents)