metrics: Initial integration of Prometheus metrics (#3709)

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

metrics.go

@@ -0,0 +1,32 @@
+package caddy
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+// define and register the metrics used in this package.
+func init() {
+	prometheus.MustRegister(prometheus.NewBuildInfoCollector())
+
+	const ns, sub = "caddy", "admin"
+
+	adminMetrics.requestCount = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: ns,
+		Subsystem: sub,
+		Name:      "http_requests_total",
+		Help:      "Counter of requests made to the Admin API's HTTP endpoints.",
+	}, []string{"handler", "path", "code", "method"})
+	adminMetrics.requestErrors = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: ns,
+		Subsystem: sub,
+		Name:      "http_request_errors_total",
+		Help:      "Number of requests resulting in middleware errors.",
+	}, []string{"handler", "path", "method"})
+}
+
+// adminMetrics is a collection of metrics that can be tracked for the admin API.
+var adminMetrics = struct {
+	requestCount  *prometheus.CounterVec
+	requestErrors *prometheus.CounterVec
+}{}