feat: allow to disable '/metrics' endpoints for local stats (#3945)

Seem the "/metrics" endpoint that is source of confusion as people tends
to believe we collect telemetry data just because we import
"opentelemetry", however it is still a good idea to allow to disable
even local metrics if not really required.

See also: https://github.com/mudler/LocalAI/issues/3942

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>