processors.rename - weird behaviour in regards to "tags"-option

[knollet]

Relevant telegraf.conf with rename processor

[[inputs.mock]]    # this input is always the same

  metric_name = "mock"
  [[inputs.mock.constant]]
    name = "constant"
    value = 123

[[processors.rename]]
namepass = ["mock"]
tags = { "test" = "doesnt_work" }     # <-------- doesn't work but no error

Logs from Telegraf

$ telegraf -config tags.conf -test -test-wait 5 -debug
...
> mock,host=myhost constant=123i 1755612972000000000

The tags option seems to exist, though, as I get an error if I try to use an option that doesn't exist:

[[inputs.mock]]    # this input is always the same

  metric_name = "mock"
  [[inputs.mock.constant]]
    name = "constant"
    value = 123

[[processors.rename]]
namepass = ["mock"]
thisdoesntexist = { "test" = "doesnt_work" }     # <------------ error!

output:

$ telegraf -config tags.conf -test -test-wait 5 -debug

2025-08-19T14:20:39Z I! Loading config: tags.conf
2025-08-19T14:20:39Z E! loading config file tags.conf failed: plugin processors.rename: line 8: configuration specified the fields ["thisdoesntexist"], but they were not used; this is either a typo or this config option does not exist in this version

The override-processor behaves as expected, though:

[[inputs.mock]]    # this input is always the same

  metric_name = "mock"
  [[inputs.mock.constant]]
    name = "constant"
    value = 123

[[processors.override]]
namepass = ["mock"]
tags = { "test" = "works" }

output:

$ telegraf -config tags.conf -test -test-wait 5 -debug
...
> mock,host=myhost,test=works constant=123i 1755613379000000000

System info

Telegraf 1.34.4 (git: HEAD@e7ce1e1e), RHEL 9

Steps to reproduce

Use "tags" option with rename-processor.

Expected behavior

Expect that the tags are added to the metric

Actual behavior

Observe that the tags aren't added to the metric