TUN-8737: update metrics server port selection

## Summary
Update how metrics server binds to a listener by using a known set of ports whenever the default address is used with the fallback to a random port in case all address are already in use. The default address changes at compile time in order to bind to a different default address when the final deliverable is a docker image.

Refactor ReadyServer tests.

Closes TUN-8737

metrics/metrics.go

@@ -10,6 +10,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/facebookgo/grace/gracenet"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/rs/zerolog"
@@ -21,6 +22,34 @@ const (
 	defaultShutdownTimeout = time.Second * 15
 )
 
+// This variable is set at compile time to allow the default local address to change.
+var Runtime = "host"
+
+func GetMetricsDefaultAddress(runtimeType string) string {
+	// When issuing the diagnostic command we may have to reach a server that is
+	// running in a virtual enviroment and in that case we must bind to 0.0.0.0
+	// otherwise the server won't be reachable.
+	switch runtimeType {
+	case "virtual":
+		return "0.0.0.0:0"
+	default:
+		return "localhost:0"
+	}
+}
+
+// GetMetricsKnownAddresses returns the addresses used by the metrics server to bind at
+// startup time to allow a semi-deterministic approach to know where the server is listening at.
+// The ports were selected because at the time we are in 2024 and they do not collide with any
+// know/registered port according https://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers.
+func GetMetricsKnownAddresses(runtimeType string) [5]string {
+	switch Runtime {
+	case "virtual":
+		return [5]string{"0.0.0.0:20241", "0.0.0.0:20242", "0.0.0.0:20243", "0.0.0.0:20244", "0.0.0.0:20245"}
+	default:
+		return [5]string{"localhost:20241", "localhost:20242", "localhost:20243", "localhost:20244", "localhost:20245"}
+	}
+}
+
 type Config struct {
 	ReadyServer         *ReadyServer
 	QuickTunnelHostname string
@@ -65,6 +94,42 @@ func newMetricsHandler(
 	return router
 }
 
+// CreateMetricsListener will create a new [net.Listener] by using an
+// known set of ports when the default address is passed with the fallback
+// of choosing a random port when none is available.
+//
+// In case the provided address is not the default one then it will be used
+// as is.
+func CreateMetricsListener(listeners *gracenet.Net, laddr string) (net.Listener, error) {
+	if laddr == GetMetricsDefaultAddress(Runtime) {
+		// On the presence of the default address select
+		// a port from the known set of addresses iteratively.
+		addresses := GetMetricsKnownAddresses(Runtime)
+		for _, address := range addresses {
+			listener, err := listeners.Listen("tcp", address)
+			if err == nil {
+				return listener, nil
+			}
+		}
+
+		// When no port is available then bind to a random one
+		listener, err := listeners.Listen("tcp", laddr)
+		if err != nil {
+			return nil, fmt.Errorf("failed to listen to default metrics address: %w", err)
+		}
+
+		return listener, nil
+	}
+
+	// Explicitly got a local address then bind to it
+	listener, err := listeners.Listen("tcp", laddr)
+	if err != nil {
+		return nil, fmt.Errorf("failed to bind to address (%s): %w", laddr, err)
+	}
+
+	return listener, nil
+}
+
 func ServeMetrics(
 	l net.Listener,
 	ctx context.Context,