Cannot access @metadata of dead_letter_queue in 7.6.0

[roflmao]

dead_letter_queue {
path => "/usr/share/logstash/data/dead_letter_queue/"
pipeline_id => "main"
commit_offsets => true
}

Trying to do stdout { codec => rubycodec { metadata => true } fails logstash from starting:

│ logstash [2020-02-27T12:35:48,091][ERROR][logstash.agent ] Failed to execute action {:action=>LogStash::PipelineAction::Create/pipeline_id:main, :exception=>"LogStash::ConfigurationError", :message=>"Expected one of [A-Za-z0-9_-], [ \t\r\ │
│ \n], "#", "{", [A-Za-z0-9_], "}" at line 3, column 23 (byte 44) after output {\n stdout { \n codec => rubydebug", :backtrace=>["/usr/share/logstash/logstash-core/lib/logstash/compiler.rb:47:in compile_imperative'", "/usr/share/logstash/log │ │ stash-core/lib/logstash/compiler.rb:55:in compile_graph'", "/usr/share/logstash/logstash-core/lib/logstash/compiler.rb:17:in block in compile_sources'", "org/jruby/RubyArray.java:2580:in map'", "/usr/share/logstash/logstash-core/lib/logstash/compi │
│ ler.rb:14:in compile_sources'", "org/logstash/execution/AbstractPipelineExt.java:161:in initialize'", "org/logstash/execution/JavaBasePipelineExt.java:47:in initialize'", "/usr/share/logstash/logstash-core/lib/logstash/java_pipeline.rb:27:in init │
│ ialize'", "/usr/share/logstash/logstash-core/lib/logstash/pipeline_action/create.rb:36:in execute'", "/usr/share/logstash/logstash-core/lib/logstash/agent.rb:326:in block in converge_state'"]}
│ logstash [2020-02-27T12:35:49,419][ERROR][logstash.agent ] Failed to execute action {:action=>LogStash::PipelineAction::Create/pipeline_id:main, :exception=>"LogStash::ConfigurationError", :message=>"Expected one of [A-Za-z0-9_-], [ \t\r\ │
│ \n], "#", "{", [A-Za-z0-9_], "}" at line 3, column 23 (byte 44) after output {\n stdout { \n codec => rubydebug", :backtrace=>["/usr/share/logstash/logstash-core/lib/logstash/compiler.rb:47:in compile_imperative'", "/usr/share/logstash/log │ │ stash-core/lib/logstash/compiler.rb:55:in compile_graph'", "/usr/share/logstash/logstash-core/lib/logstash/compiler.rb:17:in block in compile_sources'", "org/jruby/RubyArray.java:2580:in map'", "/usr/share/logstash/logstash-core/lib/logstash/compi │
│ ler.rb:14:in compile_sources'", "org/logstash/execution/AbstractPipelineExt.java:161:in initialize'", "org/logstash/execution/JavaBasePipelineExt.java:47:in initialize'", "/usr/share/logstash/logstash-core/lib/logstash/java_pipeline.rb:27:in init │
│ ialize'", "/usr/share/logstash/logstash-core/lib/logstash/pipeline_action/create.rb:36:in execute'", "/usr/share/logstash/logstash-core/lib/logstash/agent.rb:326:in block in converge_state'"]}

While stdout { codec => rubydebug } without the metadata part works flawlessly (but doesnt show metadata)

Mutating the field into a var

mutate {
add_field => { "reason" => "%{[@metadata][dead_letter_queue][reason]}" }
}

gives this field:

"reason":"%{[@metadata][dead_letter_queue][reason]}"

[roflmao]

To me its not obvious how i can make use of the dlq if i cannot find the reason to why they are put there in the first place.