Fix warning on nil tag

[cleroymira-unabiz]

Hi,

This aims at fixing warning we see in logs:

{"time":"2025-06-23","level":"warn","message":"send an error event stream to @ERROR: error_class=NoMethodError error=\"undefined method `split' for nil:NilClass\\n\\n          tags = tag.split('.')\\n                    ^^^^^^\" location=\"/fluentd/vendor/bundle/ruby/3.1.0/gems/fluent-plugin-prometheus-2.1.0/lib/fluent/plugin/prometheus/placeholder_expander.rb:42:in `build_tag'\" tag=nil","worker_id":0

I hope it is not too naive.

Thanks!

[Watson1978]

Can you add test cases?

[daipom]

Thanks!
It appears that it is usually impossible for tag to be nil.
Can you please provide a simple way to reproduce that error?

[cleroymira-unabiz]

Hi,

Thanks! It appears that it is usually impossible for tag to be nil. Can you please provide a simple way to reproduce that error?

It comes from a misconfiguration: we forgot "key" in a

<filter message>
  @type prometheus
  <metric>
    name message_foo_counter
    type counter
    desc The total number of foo in message.
    #key foo
  </metric>
</filter>