[Bug]: Pub/Sub sources incompatible with Python MapToFields

[Polber]

What happened?

ReadFromPubSub is incompatible with Python MapToFields (or at least I cannot see how to resolve the type errors). A minimal example:

pipeline:
  transforms:
    - type: ReadFromPubSub
      name: Source
      config:
        topic: projects/${PROJECT}/topics/${TOPIC_NAME}
        format: json
        schema:
          type: object
          properties:
            foo: {type: string}
    - type: MapToFields
      name: Transform
      config:
        language: python
        append: true
        fields:
          bar:
            expression: "'baz'"
            output_type: string
      input: Source
    - type: WriteToBigQuery
      name: Sink
      config:
        table: ${PROJECT}:${DATASET}.${TABLE}
        numStreams: 1
      input: Transform

This fails with

Failed to decode Schema due to an error decoding Field proto:

name: "foo"
type {
  nullable: true
  logical_type {
    urn: "beam:logical:pythonsdk_any:v1"
  }
}

despite the fact that foo's type is provided in the Pub/Sub schema

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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[Polber]

.take-issue