Metricbeat HTTP module - custom request and fetch headers only

Hello,
i'm using metricbeat / elasticsearch / kibana.
My goal is to know if the server is UP : i'm planning to do a request (HEAD request will be perfect) then process the headers (and filtering them maybe in logstash?) and check f the received response status is 200 ==> so that means server is UP.

I'm facing some issues with metricbeat configuration file.
The HTTP module is enabled and has this conf :

    - module: http
      metricsets:
        - json
      period: 10s
      hosts: ["localhost:3000"]
      namespace: "json_namespace"
      path: "/"
      method: "HEAD"
      response.enabled: true

using this conf => i got an error.message : unexpected end of JSON input

I think it's because HEAD returns an empty body and parsing an empty body gave this error.

I tried using an other conf :

  - module: http
    metricsets:
      - json
    period: 10s
    hosts: ["localhost:3000"]
    namespace: "json_namespace"
    path: "/"
    body: ""
    method: "GET"
    response.enabled: true

I got this error : invalid character '<' looking for beginning of value

I think it's because < is a starting of an html page ! and that could not be parsed in json!

I'm stuck here ! I ll be glad if you can help me figure it out !
Thank you very much!

I think in order to understand if you can use json_namespace could you please post an anonymized output of http page: http://localhost:3000
-> or is it
https://localhost:3000
?