[CT-788] Support disabling metrics + exposures

[jtcohen6]

Pretty simple:

# models/specific_file.yml

metrics:
  - name: disabled_metric
    config:
      enabled: false
  
exposures:
  - name: disabled_exposure
    config:
      enabled: false

# dbt_project.yml

metrics:
  +enabled: false

exposures:
  +enabled: false

Expectations:

• Moved to disabled section of manifest.json
• Needs to work with partial parsing
• If a disabled metric is referenced via metric(), it should raise an error at the end of parsing (just as ref() or source() do)

Grouping these together because I believe it would require touching the same code paths!

[emmyoop]

Need to make sure we loop over the right things anywhere we're looking at enabled nodes to ensure metrics/exposures are included.