My example Logstash pipeline

input { pipeline { address => my_main_pipeline } }
filter {
  mutate {
    copy => { "[log][file][path]" => "log_file_path" }
  }
}
output {
  elasticsearch {
    index => "i-my_index-%{[fields][project]}-%{[fields][instance]}-%{[fields][tenant]}-%{[fields][context]}-%{[fields][subcontext]}-%{+yyyy.MM.dd}"
    cloud_id => "${MY_ELASTIC_CLOUD_ID}"
    cloud_auth => "${MY_ELASTIC_CLOUD_AUTH}"
  }
  if [fields][subcontext] == "my_metrics" {
    file {
      path => "<PATH TO USE. DESIRABLY, I'D USE [log][file][path] HERE, BUT THE PATH ISSUE IS MAKING IT IMPOSSIBLE FOR ME>"
	  codec => line { format => "{\"Timestamp\": \"%{Timestamp}\", \"Timezone\": \"%{Timezone}\", \"Body\": %{Body} "}
    }
  }
}