DataPrepper monitoring (how much is overloaded)

Data Prepper
1

Versions (relevant - OpenSearch/Dashboard/Server OS/Browser):
DataPrepper: 2.6.2
OpenSearch: 2.11.1

Describe the issue:
We would like to monitor (=collect internal metrics from the program) DataPrepper. In the documentation I found, that they should be available in Prometheus format on the URL http://localhost:4900/metrics/sys. When I call the endpoint via curl directly from the container, I got nothing:

curl -kIlv http://localhost:4900/metrics/sys
* Host localhost:4900 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying [::1]:4900...
* connect to ::1 port 4900 from ::1 port 59918 failed: Connection refused
*   Trying 127.0.0.1:4900...
* Connected to localhost (127.0.0.1) port 4900
> HEAD /metrics/sys HTTP/1.1
> Host: localhost:4900
> User-Agent: curl/8.5.0
> Accept: */*
>
* Empty reply from server
* Closing connection
curl: (52) Empty reply from server

Configuration:

pipelines.yaml:
===
  entry-pipeline:
    workers: 4
    delay: "100"
    source:
      otel_trace_source:
        ssl: true
        sslKeyCertChainFile: "/usr/share/data-prepper/server.crt"
        sslKeyFile: "/usr/share/data-prepper/server.key"
        port: 21890
        authentication:
          http_basic:
            username: REDACTED
            password: REDACTED
    buffer:
      bounded_blocking:
        buffer_size: 1024
        batch_size: 256
    sink:
      - pipeline:
          name: "raw-pipeline"
      - pipeline:
          name: "service-map-pipeline"

  raw-pipeline:
    source:
      pipeline:
        name: "entry-pipeline"
    processor:
      - otel_traces:
    sink:
      - opensearch:
          # index_type: trace-analytics-raw
          index_type: custom
          index: otel-v1-apm-span
          hosts: [ https://REDACTED.svc.cluster.local:9200 ]
          cert: "/usr/share/data-prepper/root-ca.pem"
          template_file: "/usr/share/data-prepper/otel-span-index-template.json"
          ism_policy_file: "/usr/share/data-prepper/otel-span-ism-policy.json"
          username: REDACTED
          password: REDACTED

  service-map-pipeline:
    delay: "100"
    source:
      pipeline:
        name: "entry-pipeline"
    processor:
      - service_map:
    sink:
      - opensearch:
          index_type: trace-analytics-service-map
          hosts: [ https://REDACTED.svc.cluster.local:9200 ]
          cert: "/usr/share/data-prepper/root-ca.pem"
          username: REDACTED
          password: REDACTED

data-prepper-config.yaml
===
ssl: false

I would like to kindly ask you what I do wrong? By the documentation the metrics endpoint should be active by default. In the logs I not spot any strange or more information, that can push me forward.

Thank you very much for your time and effort!

Edit: In documentation in the repository is mentioned different metrics endpoint metrics/prometheus. Unfortunately, the endpoint give me same empty response.