Output HTTP: Problem to send @metadata from one pipeline into another

[junchao]

I am trying to send the value of [@metadata][usertag] from one pipeline 1 to pipeline 2.

I tried to parse the value using "headers" setting but the value parsed is the string: "%{[@metadata][usertag]}" and not the variable value itself.

Pipeline 1 output configuration:

output {
    http {
        url => 'http://127.0.0.1:5049'
        http_method => post
        headers => {
            "usertag" => "%{[@metadata][usertag]}"
        }
        retry_non_idempotent => true
        format => json_batch
        http_compression => true
    }
)

Pipeline 2 input configuration:

input {
    http {
        id => output
        port => 5049
        additional_codecs => { "application/json" => "json_lines" }
    }
}

I understand that I can use mutate add_field in the filter block to declare a new variable and it will be sent over from pipeline 1 to pipeline 2 right away, however, we are not supposed to touch the filter block developed by other developers from other companies. Hence, I am only limited to the output block.

Thanks in advanced for the help.