Added tests to ensure that libprocess metrics are alphabetical.
Added tests to ensure that libprocess metrics are alphabetical.

For human readability, we ensure that metrics are ordered
alphabetically in the JSON response.

Review: https://reviews.apache.org/r/65012

abudnik/mesos,gsantovena/mesos,asamerh4/mesos,kaysoky/mesos,andschwa/mesos,gsantovena/mesos,reneploetz/mesos,asamerh4/mesos,jpeach/mesos,jpeach/mesos,jpeach/mesos,andschwa/mesos,chhsia0/mesos,dforsyth/mesos,andschwa/mesos,chhsia0/mesos,kaysoky/mesos,asamerh4/mesos,reneploetz/mesos,Gilbert88/mesos,dforsyth/mesos,abudnik/mesos,andschwa/mesos,abudnik/mesos,dforsyth/mesos,abudnik/mesos,dforsyth/mesos,reneploetz/mesos,jpeach/mesos,reneploetz/mesos,chhsia0/mesos,abudnik/mesos,asamerh4/mesos,chhsia0/mesos,Gilbert88/mesos,asamerh4/mesos,asamerh4/mesos,chhsia0/mesos,jpeach/mesos,abudnik/mesos,dforsyth/mesos,gsantovena/mesos,Gilbert88/mesos,dforsyth/mesos,Gilbert88/mesos,kaysoky/mesos,reneploetz/mesos,Gilbert88/mesos,andschwa/mesos,reneploetz/mesos,andschwa/mesos,Gilbert88/mesos,gsantovena/mesos,jpeach/mesos,kaysoky/mesos,jpeach/mesos,gsantovena/mesos,asamerh4/mesos,kaysoky/mesos,dforsyth/mesos,kaysoky/mesos,gsantovena/mesos,kaysoky/mesos,chhsia0/mesos,chhsia0/mesos,Gilbert88/mesos,andschwa/mesos,gsantovena/mesos,reneploetz/mesos,abudnik/mesos