Fix Prometheus metrics

Steps to reproduce:

1. start Nuxeo by using the attached Dockerfile (it contains the updated version of simpleclient jar)
2. run the curl command and observe the output => nothing is printed !

    curl -v localhost:9090
   *   Trying 127.0.0.1:9090...
   * Connected to localhost (127.0.0.1) port 9090 (#0)
   > GET / HTTP/1.1
   > Host: localhost:9090
   > User-Agent: curl/7.76.1
   > Accept: */*
   >
   * Mark bundle as not supporting multiuse
   < HTTP/1.1 200 OK
   < Date: Thu, 04 Jan 2024 12:51:16 GMT
   < Transfer-encoding: chunked
   < Content-type: text/plain; version=0.0.4; charset=utf-8
   <
   * Connection #0 to host localhost left intact
   

Expected behavior: when running the curl command, the output should be

 curl localhost:9090
# HELP oc_worker_spans_dropped Number of spans dropped by the exporter thread.
# TYPE oc_worker_spans_dropped counter
oc_worker_spans_dropped 0.0
# HELP oc_worker_spans_pushed Number of spans pushed by the exporter thread to the exporter.
# TYPE oc_worker_spans_pushed counter
oc_worker_spans_pushed 0.0
# HELP oc_worker_spans_referenced Current number of spans referenced by the exporter thread.
# TYPE oc_worker_spans_referenced gauge
oc_worker_spans_referenced 0.0