feat: move metrics server config to the policy file (#1572)

* feat(config): add metrics bind config to policy file with flag hack

Signed-off-by: Xe Iaso <me@xeiaso.net>

* feat(internal): move SetupListener from main

Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix(main): use internal.SetupListener

Signed-off-by: Xe Iaso <me@xeiaso.net>

* fix(config): add metrics socket mode

Signed-off-by: Xe Iaso <me@xeiaso.net>

* feat: move metrics server to a dedicated package

Signed-off-by: Xe Iaso <me@xeiaso.net>

* doc: add metrics server configuration docs

Signed-off-by: Xe Iaso <me@xeiaso.net>

* doc(default-config): add vague references to metrics server

Signed-off-by: Xe Iaso <me@xeiaso.net>

* chore: spelling

Signed-off-by: Xe Iaso <me@xeiaso.net>

---------

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/config/config.go

@@ -334,6 +334,7 @@ type fileConfig struct {
 	DNSBL       bool                `json:"dnsbl"`
 	DNSTTL      DnsTTL              `json:"dns_ttl"`
 	Logging     *Logging            `json:"logging"`
+	Metrics     *Metrics            `json:"metrics,omitempty"`
 }
 
 func (c *fileConfig) Valid() error {
@@ -375,6 +376,12 @@ func (c *fileConfig) Valid() error {
 		}
 	}
 
+	if c.Metrics != nil {
+		if err := c.Metrics.Valid(); err != nil {
+			errs = append(errs, err)
+		}
+	}
+
 	if len(errs) != 0 {
 		return fmt.Errorf("config is not valid:\n%w", errors.Join(errs...))
 	}
@@ -417,6 +424,7 @@ func Load(fin io.Reader, fname string) (*Config, error) {
 		StatusCodes: c.StatusCodes,
 		Store:       c.Store,
 		Logging:     c.Logging,
+		Metrics:     c.Metrics,
 	}
 
 	if c.OpenGraph.TimeToLive != "" {
@@ -508,6 +516,7 @@ type Config struct {
 	Logging     *Logging
 	DNSBL       bool
 	DNSTTL      DnsTTL
+	Metrics     *Metrics
 }
 
 func (c Config) Valid() error {