`is_metric` is always false

[MiConnell]

the current metric logic looks for expression and refs, but semantic models now use expr and depends_on.nodes so any metrics defined in the semantic models definition shows N for is_metric.

{
    "name": "price",
    "resource_type": "metric",
    "package_name": "project_name",
    "..."
    "type_params": {
        "..."
        "expr": "price",
    },
   "..."
    "depends_on": {
        "macros": [],
        "nodes": ["semantic_model.project_name.fact_sales"],
    },
    "refs": [],
}