Set TLS Handshake logs to debug

This adds a logDebug, which if set true, prints debug-level logs.

The logs from the http.Server are quite noisy from random internet scans, so
set them to debug.

Since we still want to see errors from our own getCertificate, add a logging
wrapper for it at the warn level.

certs/certs.go

@@ -88,6 +88,16 @@ func isACME(info *tls.ClientHelloInfo) bool {
 
 // GetCertificate implements the interface required by tls.Config
 func (c *CertManager) GetCertificate(info *tls.ClientHelloInfo) (*tls.Certificate, error) {
+	cert, err := c.get(info)
+	if err != nil {
+		slog.Warn("getting certificate", slog.String("error", err.Error()))
+	}
+
+	return cert, err
+}
+
+// get is the core of GetCertificate, which wraps this for observability
+func (c *CertManager) get(info *tls.ClientHelloInfo) (*tls.Certificate, error) { //nolint:funcorder
 	sni := info.ServerName
 
 	c.mu.Lock()

config/config.go

@@ -96,6 +96,9 @@ type Config struct {
 
 	// TextTemplate overrides the default plain text webpage
 	TextTemplate string
+
+	// LogDebug enables debug level logs when set to true
+	LogDebug bool
 }
 
 // Site configures a particular site.

integration/test-certs-site-config.json

@@ -15,5 +15,6 @@
     "directory": "https://pebble:14000/dir",
     "termsOfServiceAgreed": true
   },
-  "dataDir": "/data"
+  "dataDir": "/data",
+  "logDebug": true
 }

main.go

@@ -21,7 +21,9 @@ import (
 )
 
 func run(args []string) error {
-	slog.SetDefault(slog.New(slog.NewJSONHandler(os.Stdout, nil)))
+	logLevel := &slog.LevelVar{}
+	logLevel.Set(slog.LevelInfo)
+	slog.SetDefault(slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{Level: logLevel})))
 
 	fs := flag.NewFlagSet(args[0], flag.ExitOnError)
 	cfgPath := fs.String("config", "config.json", "path to json config file")
@@ -40,6 +42,10 @@ func run(args []string) error {
 		return fmt.Errorf("loading config: %w", err)
 	}
 
+	if cfg.LogDebug {
+		logLevel.Set(slog.LevelDebug)
+	}
+
 	store, err := storage.New(cfg.DataDir)
 	if err != nil {
 		return fmt.Errorf("creating storage: %w", err)

server/server.go

@@ -26,9 +26,12 @@ func Run(ctx context.Context, cfg *config.Config, getCert GetCertificateFunc) er
 		return err
 	}
 
+	logger := slog.NewLogLogger(slog.Default().Handler(), slog.LevelDebug)
+
 	srv := http.Server{
-		Addr:    cfg.ListenAddr,
-		Handler: handler,
+		Addr:     cfg.ListenAddr,
+		Handler:  handler,
+		ErrorLog: logger,
 
 		IdleTimeout:       timeout,
 		ReadHeaderTimeout: timeout,