[#4071] Add metrics feature (#4235)

* first cut at supporting metrics definitions

* teach dbt about metrics

* wip

* support partial parsing for metrics

* working on tests

* Fix some tests

* Add partial parsing metrics test

* Fix some more tests

* Update CHANGELOG.md

* Fix partial parsing yaml file to correct model syntax

Co-authored-by: Drew Banin <drew@fishtownanalytics.com>

automatic commit by git-black, original commits:
  31c88f9
  6c6649f
  726004b
  855419d
  c69f28e
  d60f6bc

core/dbt/contracts/graph/manifest.py

@@ -661,19 +661,7 @@ def build_flat_graph(self):
             "exposures": {k: v.to_dict(omit_none=False) for k, v in self.exposures.items()},
             "metrics": {k: v.to_dict(omit_none=False) for k, v in self.metrics.items()},
             "nodes": {k: v.to_dict(omit_none=False) for k, v in self.nodes.items()},
-            },
-            'metrics': {
-                k: v.to_dict(omit_none=False)
-                for k, v in self.metrics.items()
-            },
-            'nodes': {
-                k: v.to_dict(omit_none=False)
-                for k, v in self.nodes.items()
-            },
-            'sources': {
-                k: v.to_dict(omit_none=False)
-                for k, v in self.sources.items()
-            }
+            "sources": {k: v.to_dict(omit_none=False) for k, v in self.sources.items()},
         }
 
     def build_disabled_by_file_id(self):