Custom Metric Table dbt transform example

[thomas-gerber]

Is your feature request related to a problem? Please describe.
Users may want to generate custom metric tables to be leveraged in Metabase.

Describe the solution you'd like
dbt transform that creates a Task Status Duration table that for each task and non-final status in the Task table, computes the duration the task was in that status.

Describe alternatives you've considered
Saved Questions in Metabase. While that is possible, it may be desirable to transform the data outside of the query path, for performance and UX considerations (sparing the users to browse both Saved Questions AND the tables).

Additional context
This transform is a fundamental entry in our transform catalog. We will automate the orchestration and execution of those dbt transforms as part of #71 .