metrics: scope metrics to active config, add optional per-host metrics (#6531)

* Add per host config

* Pass host label when option is enabled

* Test per host enabled

* metrics: scope metrics per loaded config

* doc and linter

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

* inject the custom registry into the admin handler

Co-Authored-By: Dave Henderson <dhenderson@gmail.com>

* remove `TODO` comment

* fixes

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

* refactor to delay metrics admin handler provision

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

---------

Signed-off-by: Mohammed Al Sahaf <msaa1990@gmail.com>
Co-authored-by: Hussam Almarzooq <me@hussam.io>
Co-authored-by: Dave Henderson <dhenderson@gmail.com>

modules/metrics/adminmetrics.go

@@ -15,8 +15,11 @@
 package metrics
 
 import (
+	"errors"
 	"net/http"
 
+	"github.com/prometheus/client_golang/prometheus"
+
 	"github.com/caddyserver/caddy/v2"
 )
 
@@ -29,7 +32,11 @@ func init() {
 // is permanently mounted to the admin API endpoint at "/metrics".
 // See the Metrics module for a configurable endpoint that is usable if the
 // Admin API is disabled.
-type AdminMetrics struct{}
+type AdminMetrics struct {
+	registry *prometheus.Registry
+
+	metricsHandler http.Handler
+}
 
 // CaddyModule returns the Caddy module information.
 func (AdminMetrics) CaddyModule() caddy.ModuleInfo {
@@ -39,17 +46,28 @@ func (AdminMetrics) CaddyModule() caddy.ModuleInfo {
 	}
 }
 
+// Provision -
+func (m *AdminMetrics) Provision(ctx caddy.Context) error {
+	m.registry = ctx.GetMetricsRegistry()
+	if m.registry == nil {
+		return errors.New("no metrics registry found")
+	}
+	m.metricsHandler = createMetricsHandler(nil, false, m.registry)
+	return nil
+}
+
 // Routes returns a route for the /metrics endpoint.
 func (m *AdminMetrics) Routes() []caddy.AdminRoute {
-	metricsHandler := createMetricsHandler(nil, false)
-	h := caddy.AdminHandlerFunc(func(w http.ResponseWriter, r *http.Request) error {
-		metricsHandler.ServeHTTP(w, r)
-		return nil
-	})
-	return []caddy.AdminRoute{{Pattern: "/metrics", Handler: h}}
+	return []caddy.AdminRoute{{Pattern: "/metrics", Handler: caddy.AdminHandlerFunc(m.serveHTTP)}}
+}
+
+func (m *AdminMetrics) serveHTTP(w http.ResponseWriter, r *http.Request) error {
+	m.metricsHandler.ServeHTTP(w, r)
+	return nil
 }
 
 // Interface guards
 var (
+	_ caddy.Provisioner = (*AdminMetrics)(nil)
 	_ caddy.AdminRouter = (*AdminMetrics)(nil)
 )