Metrics server without HTTPS

[riptl]

Problem

The metrics server cannot be started without an HTTPS certificate.
nimiq: Cannot provide metrics when running without a certificate

This makes it impossible to use the metrics server behind a reverse proxy.

Feature Request

Add a tls: bool field to metrics server config (default true) to make it work without a cert.
Maybe also an allowed-ips/cidrs list to avoid accidental web exposure.