Function get_fragment_vertex_to_relation_id_map

pub async fn get_fragment_vertex_to_relation_id_map(
    __arg0: Extension<Service>,
) -> Result<Json<FragmentVertexToRelationMap>, DashboardError>

In the ddl backpressure graph, we want to compute the backpressure between relations. So we need to know which are the fragments which are connected to external relations. These fragments form the vertices of the graph. We can get collection of backpressure values, keyed by vertex_id-vertex_id. This function will return a map of fragment vertex id to relation id. We can convert vertex_id-vertex_id to relation_id-relation_id using that. Finally, we have a map of relation_id-relation_id to backpressure values.