metrics: add option metrics.addr for ipv6 support

`prometheus_client` supports IPv6 addresses for a while now[1]. This
isn't used here however because the default 0.0.0.0 is used. Added an
option `metrics.addr` that makes this configurable.

[1] prometheus/client_python@39d2360

mautrix/util/program.py

@@ -188,6 +188,7 @@ def start_prometheus(self) -> None:
         try:
             enabled = self.config["metrics.enabled"]
             listen_port = self.config["metrics.listen_port"]
+            addr = self.config.get("metrics.addr", '0.0.0.0')
         except KeyError:
             return
         if not enabled:
@@ -197,7 +198,7 @@ def start_prometheus(self) -> None:
                 "Metrics are enabled in config, but prometheus_client is not installed"
             )
             return
-        prometheus.start_http_server(listen_port)
+        prometheus.start_http_server(listen_port, addr=addr)
 
     def _run(self) -> None:
         signal.signal(signal.SIGINT, signal.default_int_handler)