Avro union fields listed in avro_tags do not get converted to tags

[fajpunk]

Relevant telegraf.conf

[[ inputs.file ]]
  files = ["./testcases/union-nullable-tag/message.json"]
  data_format = "avro"

  avro_format = "json"
  avro_measurement = "Switch"
  avro_tags = ["switch_wwn", "some_union_in_a_tag"]
  avro_fields = ["up_time", "cpu_utilization", "memory_utilization", "statistics_collection_time"]
  avro_timestamp = "statistics_collection_time"
  avro_timestamp_format = "unix_ms"
  avro_union_mode = "nullable"
  avro_schema = '''
        {
                "namespace": "com.brocade.streaming",
                "name": "fibrechannel_switch_statistics",
                "type": "record",
                "version": "1",
                "fields": [
                        {"name": "some_union_in_a_tag", "type": ["null", "string"], "default": null, "doc": "Some union that is used in a tag"},
                        {"name": "switch_wwn", "type": "string", "doc": "WWN of the Physical Switch."},
                        {"name": "statistics_collection_time", "type": "long", "doc": "Epoch time when statistics is collected."},
                        {"name": "up_time", "type": "long", "doc": "Switch Up Time (in hundredths of a second)"},
                        {"name": "cpu_utilization", "type": ["null","float"], "default": null, "doc": "CPU Utilization in %"},
                        {"name": "memory_utilization", "type": ["null", "float"], "default": null, "doc": "Memory Utilization in %"}
                ]
        }
  '''

Logs from Telegraf

2024-12-06T19:43:12Z W! [parsers.avro::file] Could not convert map[string:some_value] to string for tag "some_union_in_a_tag": type "map[string]interface {}" unsupported

System info

docker.io/library/telegraf:1.30.2-alpine

Docker

No response

Steps to reproduce

1. Configure telegraf with an avro input (binary, json, whatever, it doesn't matter)
2. Configure telegraf with an avro schema that includes a field with a union type (schema can be specified in the config, or come from the schema registry, doesn't matter)
3. Put that union field in avro_tags in the input configuration
4. Try to ingest an avro-encoded message that matches that schema

Expected behavior

An InfluxDB metric is inserted with the value in avro_tags as a tag on the metric

Actual behavior

The value does not get added to the metric as a tag

Additional info

No response