label handling

[wey-gu]

Add label expression as below:

---
# If vertex id is string type, remap_vertex_id must be true.
remap_vertex_id: True
space: yelp
# str or int
vertex_id_type: int
vertex_tags:
  - name: review
    label:
      name: is_fraud
      properties:
        - name: is_fraud
          type: int
          nullable: False
      filter:
        type: value
    features:
      - name: f0
        properties:
          - name: f0
            type: float
            nullable: False
        filter:
          type: value
      - name: f1
        properties:
          - name: f1
            type: float
            nullable: False
        filter:
          type: value