code: public static Schema getKeySchema(Schema topicSchema) {
      Preconditions.checkArgument(topicSchema.getType() == Schema.Type.RECORD);
  
      Optional<Schema> newSchema = getKeySchemaFromRecord(topicSchema);
      if (newSchema.isPresent()) {
        return newSchema.get();
      } else {
        LOG.warn(String.format("No field in the schema of %s is annotated as primarykey. Using all fields for deduping",
            topicSchema.getName()));
        return topicSchema;
      }
    }
docs: Returns a schema composed of all fields in topicSchema whose doc match "(?i).*primarykey".
  If there's no such field, topicSchema itself will be returned.