feat(metrics): basic auth support (#1579)

* feat(internal): add basic auth HTTP middleware

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

* feat(config): add HTTP basic auth for metrics

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

* feat(metrics): wire up basic auth

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

* doc: document HTTP basic auth for metrics server

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

* chore: spelling

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

* docs(admin/policies): give people a python command

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

---------

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

internal/basicauth.go

@@ -0,0 +1,52 @@
+package internal
+
+import (
+	"crypto/sha256"
+	"crypto/subtle"
+	"fmt"
+	"log/slog"
+	"net/http"
+)
+
+// BasicAuth wraps next in HTTP Basic authentication using the provided
+// credentials. If either username or password is empty, next is returned
+// unchanged and a debug log line is emitted.
+//
+// Credentials are compared in constant time to avoid leaking information
+// through timing side channels.
+func BasicAuth(realm, username, password string, next http.Handler) http.Handler {
+	if username == "" || password == "" {
+		slog.Debug("skipping middleware, basic auth credentials are empty")
+		return next
+	}
+
+	expectedUser := sha256.Sum256([]byte(username))
+	expectedPass := sha256.Sum256([]byte(password))
+	challenge := fmt.Sprintf("Basic realm=%q, charset=\"UTF-8\"", realm)
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		user, pass, ok := r.BasicAuth()
+		if !ok {
+			unauthorized(w, challenge)
+			return
+		}
+
+		gotUser := sha256.Sum256([]byte(user))
+		gotPass := sha256.Sum256([]byte(pass))
+
+		userMatch := subtle.ConstantTimeCompare(gotUser[:], expectedUser[:])
+		passMatch := subtle.ConstantTimeCompare(gotPass[:], expectedPass[:])
+
+		if userMatch&passMatch != 1 {
+			unauthorized(w, challenge)
+			return
+		}
+
+		next.ServeHTTP(w, r)
+	})
+}
+
+func unauthorized(w http.ResponseWriter, challenge string) {
+	w.Header().Set("WWW-Authenticate", challenge)
+	http.Error(w, "Unauthorized", http.StatusUnauthorized)
+}