Metrics Sample's resource_metadata missed instance's metadata field

[Zeng Ming]

Except for meta_data name starts with "metering.", we find that discovered Instance's metadata field is missing in Metrics Sample's resource_metadata. We are not sure if it is designed or it is just a miss. This field is important to us for we want to leverage this field to populate more resource info to back-end storage(E.g. openTSDB) at method publish_samples(self, context, samples) of class publisher.PublisherBase.

For usering facing perspective, it'd be more straightforward to get known VM's FQDN & related IP for each metrics.

E.g. below is current resource_matadata of Metric "cpu_util" sample.
"resource_metadata" : {
  "cpu_number" : 1,
  "display_name" : "test06",
  "name" : "instance-00000028",
  "disk_gb" : 1,
  "image" : {
   "id" : "92cf3554-95dd-4642-b062-2eaa9533ed9f",
   "links" : [
    {
     "href" : "http://xxxx:8774/f0d4a75e924c4665aaad7b91bd9157c6/images/92cf3554-95dd-4642-b062-2eaa9533ed9f",
     "rel" : "bookmark"
    }
   ],
   "name" : "oria_image"
  },
  "ephemeral_gb" : 0,
  "host" : "073a87e7535bae59af3d4303c9763f4c751df8c436b8eea136110c15",
  "memory_mb" : 512,
  "instance_type" : "1",
  "vcpus" : 1,
  "root_gb" : 1,
  "image_ref" : "92cf3554-95dd-4642-b062-2eaa9533ed9f",
  "flavor" : {
   "name" : "m1.tiny",
   "links" : [
    {
     "href" : "http://xxxx:8774/f0d4a75e924c4665aaad7b91bd9157c6/flavors/1",
     "rel" : "bookmark"
    }
   ],
   "ram" : 512,
   "ephemeral" : 0,
   "vcpus" : 1,
   "disk" : 1,
   "id" : "1"
  },
  "OS-EXT-AZ:availability_zone" : "nova",
  "image_ref_url" : "http://xxxx:8774/f0d4a75e924c4665aaad7b91bd9157c6/images/92cf3554-95dd-4642-b062-2eaa9533ed9f"
}

However, when I looked at nova show <uuid>, I found there is a metadata attribute which saves lots of information such as vm fqdn as below:
metadata | {"project_cos": "dev", "fqdn": "xxxx-327632.xxx.com",...}

For compute resource, If we explicitly populate discovered Instance's metadata field to Metrics Sample's resource_metadata, it will be very useful.