Add descriptors for new metrics to global descriptors
Add descriptors for new metrics to global descriptors

This PR adds a couple of new descriptors to the global config for mixer (in testdata/configroot).

Notably, these are not added to the rules for any scope/subject. This is by design at this point (to allow for adding them as part of a Task on configuring metrics -- since you there is no way for adding metric descriptors currently supported).

The new metrics are:

request_size
response_size
Some of the other descriptors are also updated to support validation.

Tested with minikube (pushed a local docker image in place of mixer, updated via istioctl mixer commands).
xiaolanz/mixer,guptasu/mixer,istio/old_mixer_repo,rkpagadala/mixer,istio/old_mixer_repo,guptasu/mixer,douglas-reid/mixer,ZackButcher/mixer,ZackButcher/mixer,ZackButcher/mixer,xiaolanz/mixer,rkpagadala/mixer,istio/old_mixer_repo,jmuk/mixer,rkpagadala/mixer,jmuk/mixer,guptasu/mixer,douglas-reid/mixer,douglas-reid/mixer,jmuk/mixer,xiaolanz/mixer