Fixed a bug in the staging tasks metric.
Fixed a bug in the staging tasks metric.

Review: https://reviews.apache.org/r/25303

neilconway/mesos,verizonlabs/mesos,reneploetz/mesos,craimbert/mesos,zmalik/mesos,asamerh4/mesos,Gilbert88/mesos,reneploetz/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,abudnik/mesos,reneploetz/mesos,dforsyth/mesos,kaysoky/mesos,craimbert/mesos,zmalik/mesos,abudnik/mesos,Gilbert88/mesos,jpeach/mesos,Gilbert88/mesos,neilconway/mesos,gsantovena/mesos,andschwa/mesos,jpeach/mesos,kaysoky/mesos,chhsia0/mesos,chhsia0/mesos,neilconway/mesos,neilconway/mesos,yuquanshan/customizedMesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,andschwa/mesos,chhsia0/mesos,chhsia0/mesos,chhsia0/mesos,asamerh4/mesos,verizonlabs/mesos,kaysoky/mesos,craimbert/mesos,jpeach/mesos,kaysoky/mesos,yuquanshan/customizedMesos,jpeach/mesos,abudnik/mesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,neilconway/mesos,andschwa/mesos,asamerh4/mesos,abudnik/mesos,zmalik/mesos,abudnik/mesos,chhsia0/mesos,Aman-Jain-14/customizedMesos,chhsia0/mesos,gsantovena/mesos,asamerh4/mesos,yuquanshan/customizedMesos,Gilbert88/mesos,dforsyth/mesos,Aman-Jain-14/customizedMesos,Aman-Jain-14/customizedMesos,neilconway/mesos,reneploetz/mesos,yuquanshan/customizedMesos,jpeach/mesos,verizonlabs/mesos,craimbert/mesos,andschwa/mesos,zmalik/mesos,dforsyth/mesos,verizonlabs/mesos,verizonlabs/mesos,asamerh4/mesos,zmalik/mesos,dforsyth/mesos,abudnik/mesos,Gilbert88/mesos,reneploetz/mesos,kaysoky/mesos,craimbert/mesos,craimbert/mesos,asamerh4/mesos,andschwa/mesos,gsantovena/mesos,yuquanshan/customizedMesos,yuquanshan/customizedMesos,jpeach/mesos,asamerh4/mesos,Gilbert88/mesos,neilconway/mesos,reneploetz/mesos,craimbert/mesos,zmalik/mesos,dforsyth/mesos,reneploetz/mesos,dforsyth/mesos,abudnik/mesos,zmalik/mesos,verizonlabs/mesos,kaysoky/mesos,verizonlabs/mesos,gsantovena/mesos,gsantovena/mesos,jpeach/mesos,kaysoky/mesos,dforsyth/mesos,Aman-Jain-14/customizedMesos,andschwa/mesos,andschwa/mesos