Creating variables on grafana version 10 from influxdb v2.7 fields

Question

I am creating grafana variable from below flux query, from tags. I am using Grafana 10 and influxdb v2.7.4 and using Flux for queries.

import "influxdata/influxdb/schema"

schema.tagValues(
bucket: "Vbuck",
tag: "Service_Name"
)

I also need to create variable from influxdb fields from a specific measurement. Influxdb version is v2.7.4

I tried below bu not working.

from(bucket: "Vbuck")
  |> range(start: -30d)
  |> filter(fn: (r) => r["_measurement"] == "VMwareRawData14")
  |> group(columns: ["Policy_Type"])
  |> distinct(column: "Policy_Type")
  |> keep(columns: ["Policy_Type"])

I want to create grafana variable from FIELDS. Not TAGS.