fix: derived metrics in dbt Cloud

[betodealmeida]

There are two ways to represent derived metrics in dbt. Before dbt 1.3:

- name: some_ratio
  type: expression
  sql: "{{metric(a)}} / {{metric(b)}}"

Starting with dbt 1.3:

- name: some_ratio
  calculation_method: derived
  expression: "{{metric(a)}} / {{metric(b)}}

For some reason, the API for dbt Cloud returns a derived metric as:

{
    "name": "ratio_of_paying_customers",
    "sql": "paying_customers / total_customers",
    "type": "derived",
    "unique_id": "metric.jaffle_shop.ratio_of_paying_customers",
}

This PR fixes the is_derived function to handle this correctly.

[lilykuang]

lgtm