We need to update the serializer for the tag rates to only allow one metric per cost type. 

For example,

 ```
{
"metric":

{ "name": "node_cost_per_month", "label_metric": "Node", "label_measurement": "Currency", "label_measurement_unit": "node-month" }

,
"description": "",
"tag_rates": [
{
"tag_key": "app",
"tag_values": [
{
"unit": "USD",
"usage":

{ "unit": "USD", "usage_end": null, "usage_start": null }

,
"value": 24000.0,
"default": true,
"tag_value": "smoking",
"description": ""
}
]
}
],
"cost_type": "Infrastructure"
},
{
"metric":

{ "name": "node_cost_per_month", "label_metric": "Node", "label_measurement": "Currency", "label_measurement_unit": "node-month" }

```
 

 

If the cost_type of the second "node_cost_per_month" is infrastructure then the cost model should be rejected; however, if it is supplementary, then it should be allowed.