code: private Schema readUnion(JsonReader reader, Set<String> knownRecords) throws IOException {
      ImmutableList.Builder<Schema> unionSchemas = ImmutableList.builder();
      reader.beginArray();
      while (reader.peek() != JsonToken.END_ARRAY) {
        unionSchemas.add(read(reader, knownRecords));
      }
      reader.endArray();
      return Schema.unionOf(unionSchemas.build());
    }
docs: Constructs {@link Schema.Type#UNION UNION} type schema from the json input.
  
  @param reader The {@link JsonReader} for streaming json input tokens.
  @param knownRecords Set of record name already encountered during the reading.
  @return A {@link Schema} of type {@link Schema.Type#UNION UNION}.
  @throws java.io.IOException When fails to construct a valid schema from the input.