Authentication username and password are not interpolated

[lucabelluccini]

Logstash 7.6.2
Plugin version logstash-filter-http (1.0.2)

Summary

The http filter is not interpolating the variables for the parameters username / password.

How to reproduce

input {
    exec {
        command => 'echo "myusername mypassword something else as message"'
        interval => 30
    }
}
filter {
    grok {
        match => {
            "message" => "(?<username>.*?) (?<password>.*?) %{GREEDYDATA:message}"
        }
        overwrite => [ "message" ]
    }
    http {
        url => "https://postman-echo.com/post"
        verb => "POST"
        body_format => text
        body => "%{[message]}"
        headers => {
            "mytest" => "logstash"
        }
        user => "%{[username]}"
        password => "%{[password]}"
    }
}
output {
    stdout {
        codec => rubydebug
    }
}

Expected

The header should contain Basic bXl1c2VybmFtZTpteXBhc3N3b3Jk

Output

The header contains Basic JXtbdXNlcm5hbWVdfTole1twYXNzd29yZF19 (represents %{[username]}:%{[password]}).

Workaround

    ruby {
        init => "require 'base64'"
        code => "event.set('auth_header', 'Basic ' + Base64.strict_encode64(event.get('username')+':'+event.get('password')))"
    }
    http {
        url => "https://postman-echo.com/post"
        verb => "POST"
        body_format => text
        body => "%{[message]}"
        headers => {
            "authorization" => "%{[auth_header]}"
            "mytest" => "logstash"
        }
        remove_field => [ "auth_header"]
    }

---

From https://discuss.elastic.co/t/http-filter-plugin-authorization-as-parameter-values/232085/6