[CT-1352] Metric SQL generation breaks with dbt-core 1.3.0

[popeliam]

Describe the bug

After the dbt-core 1.3.0 release with the new v7 manifest, dbt-docs is no longer generating the correct SQL for metrics. When viewing the code section of metrics, the first line is always compiled as

select undefined(undefined)

I think the problem is that some of the metrics attributes/attribute values were renamed, specifically sql -> expression, type -> calculation_method, and type: expression -> calculation_method: derived.

In dbt-docs the metric SQL generator contains logic that still uses the sql, type, and type: expression attributes. Since sql and type are no longer defined they just get compiled as undefined.

1.3.0 did retain backwards compatibility but it did so by replacing the attributes during parsing and not retaining the old ones. This means that when using the old attribute names in a 1.3.0 project the dbt docs site still compiles as undefined(undefined)

Steps To Reproduce

dbt docs generate && dbt docs serve in a 1.3.0 dbt project that contains metrics with either the new or old attribute names.

Expected behavior

When I define metrics in a dbt 1.3.0 project, the SQL in their Code section should be compiled properly.

Screenshots and log output

The output of dbt --version:

Core:
  - installed: 1.3.0
  - latest:    1.3.0 - Up to date!

Plugins:
  - bigquery: 1.3.0 - Up to date!

Additional context

Add any other context about the problem here.

[jtcohen6]

@popeliam Thanks for opening! You're quite right. Let's get this fixed, and slated for a v1.3.1 patch release.

[popeliam]

@jtcohen6 Sure thing, thanks!