metrics: Initial integration of Prometheus metrics (#3709)

Signed-off-by: Dave Henderson <dhenderson@gmail.com>

modules/metrics/adminmetrics.go

@@ -0,0 +1,55 @@
+// Copyright 2020 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package metrics
+
+import (
+	"net/http"
+
+	"github.com/caddyserver/caddy/v2"
+)
+
+func init() {
+	caddy.RegisterModule(AdminMetrics{})
+}
+
+// AdminMetrics is a module that serves a metrics endpoint so that any gathered
+// metrics can be exposed for scraping. This module is not configurable, and
+// 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{}
+
+// CaddyModule returns the Caddy module information.
+func (AdminMetrics) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		ID:  "admin.api.metrics",
+		New: func() caddy.Module { return new(AdminMetrics) },
+	}
+}
+
+// Routes returns a route for the /metrics endpoint.
+func (m *AdminMetrics) Routes() []caddy.AdminRoute {
+	metricsHandler := createMetricsHandler(nil)
+	h := caddy.AdminHandlerFunc(func(w http.ResponseWriter, r *http.Request) error {
+		metricsHandler.ServeHTTP(w, r)
+		return nil
+	})
+	return []caddy.AdminRoute{{Pattern: "/metrics", Handler: h}}
+}
+
+// Interface guards
+var (
+	_ caddy.AdminRouter = (*AdminMetrics)(nil)
+)