Improve metrics docs & DAG viz

[drewbanin]

resolves #246

Description

This PR:

• Draws DAG edges for expression metrics (ie. a metric that depends on one or more other metrics)
• Renders labels, types (expression vs. aggregate), and SQL for each metric
• Uses the metric label rather than the name in the sidebar + DAG viz + search results
• Supports search results on metric + label name (eg. both "arpc" and "Revenue") should pull up the arpc metric

Screenshots

Expression metrics render in the DAG view

Render the name, label, and type for each metric

Example metric SQL definition:

select sum(order_total)
from {{ ref('fct_orders') }}
where is_paid = true

Checklist

• I have signed the CLA
• I have generated docs locally, and this change appears to resolve the stated issue
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

[emmyoop]

Looks good to me! I confirmed with some testing that it's behaving as expected. However, I'd love to get @stu-k's eyes on this before merging it in.

[stu-k]

Only nitpicky comments about consistency and code style, all of this seems functional as it is now 👍

[drewbanin]

@stu-k! Thank you so much for this review! I just moved this out of the draft state - lmk if there's anything else I should do before merging! :D

[stu-k]

@drewbanin As long as you tested locally and everything is working as expected, it should be good to go!

[drewbanin]

right on - I pulled these new commits down and tested metrics on local. Merging!