Added missing HostMetrics model

Added missing HostMetrics model that wraps Host.metrics OpaqueRef.

  conn = Fog::Compute.new({
    :provider => 'XenServer',
    :xenserver_url => 'xenserver-test',
    :xenserver_username => 'root',
    :xenserver_password => 'secret',
  })

  pp conn.hosts.first.metrics # => yields

  #  <Fog::Compute::XenServer::HostMetrics
  #    reference="OpaqueRef:161923b3-47e4-7f8c-8995-3030be9d58f8",
  #    uuid="00d47697-3682-1e91-154b-00116a5b7878",
  #    live=true,
  #    memory_free="3311230976",
  #    memory_total="4217688064",
  #    other_config={},
  #    last_updated=2012-12-06 21:19:59 UTC
  #  >

lib/fog/xenserver/compute.rb

@@ -36,6 +36,7 @@ module Fog
       model  :pbd
       model  :guest_metrics
       model  :vbd_metrics
+      model  :host_metrics
 
       request_path 'fog/xenserver/requests/compute'
       request :create_server

lib/fog/xenserver/models/compute/host.rb

@@ -16,7 +16,7 @@ module Fog
         attribute :allowed_operations
         attribute :enabled
         attribute :hostname
-        attribute :metrics
+        attribute :__metrics,          :aliases => :metrics
         attribute :name_description
         attribute :other_config
         attribute :__pbds,            :aliases => :PBDs
@@ -39,6 +39,12 @@ module Fog
           resident_servers
         end
 
+        def metrics
+          return nil unless __metrics
+          rec = connection.get_record(__metrics, 'host_metrics' )
+          Fog::Compute::XenServer::HostMetrics.new(rec)
+        end
+
       end
       
     end

lib/fog/xenserver/models/compute/host_metrics.rb

@@ -0,0 +1,29 @@
+require 'fog/core/model'
+
+module Fog
+  module Compute
+    class XenServer
+    
+      class HostMetrics < Fog::Model
+        # API Reference here:
+        # http://docs.vmd.citrix.com/XenServer/5.6.0/1.0/en_gb/api/?c=VM_host_metrics
+        
+        identity :reference
+        
+        attribute :uuid
+        attribute :live
+        attribute :memory_free
+        attribute :memory_total
+        attribute :other_config
+        attribute :last_updated
+        
+        def initialize(attributes = {})
+          super
+          self.last_updated = attributes[:last_updated].to_time
+        end
+        
+      end
+      
+    end
+  end
+end