metrics: move metrics up, outside servers (#6606)

* metrics: move `metrics`  up, outside `servers`

This change moves the metrics configuration from per-server level to a single config knob within the `http` app. Enabling `metrics` in any of the configured servers inside `http` enables metrics for all servers.

Fix #6604

Signed-off-by: Mohammed Al Sahaf <msaa1990@gmail.com>

* normalize domain name

---------

Signed-off-by: Mohammed Al Sahaf <msaa1990@gmail.com>

modules/caddyhttp/server.go

@@ -226,6 +226,7 @@ type Server struct {
 
 	// If set, metrics observations will be enabled.
 	// This setting is EXPERIMENTAL and subject to change.
+	// DEPRECATED: Use the app-level `metrics` field.
 	Metrics *Metrics `json:"metrics,omitempty"`
 
 	name string