[CT-2523] Begin parsing of Metrics to the Manifest as nodes

[QMalcolm]

In #7500 we defined dbt-core's representation of a Metric node. In #7456 we defined the the expected YAML representation of a Metric. This issue is to begin parsing metrics defined in YAML to Metric nodes on the Manifest. We do have a prototype of one way this could be done, #7187, if inspiration is needed.

Blocked by:

• [CT-2522] Redefine Metric node for the Manifest and update parsing #7500

[QMalcolm]

Closing this issue and instead wrapping the work to be done as part of #7500