Add doc meta data. (#4442)

### What problem does this PR solve?

#3690

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

api/db/db_models.py

@@ -760,6 +760,7 @@ class Document(DataBaseModel):
         default="")
     process_begin_at = DateTimeField(null=True, index=True)
     process_duation = FloatField(default=0)
+    meta_fields = JSONField(null=True, default={})
 
     run = CharField(
         max_length=1,
@@ -1112,3 +1113,10 @@ def migrate_db():
             )
         except Exception:
             pass
+        try:
+            migrate(
+                migrator.add_column("document", "meta_fields",
+                                    JSONField(null=True, default={}))
+            )
+        except Exception:
+            pass