changes per @ethanfrey comments

log/nop_logger.go

@@ -2,6 +2,9 @@ package log
 
 type nopLogger struct{}
 
+// Interface assertions
+var _ Logger = (*nopLogger)(nil)
+
 // NewNopLogger returns a logger that doesn't do anything.
 func NewNopLogger() Logger { return &nopLogger{} }
 
@@ -16,3 +19,11 @@ func (nopLogger) Debug(string, ...interface{}) error {
 func (nopLogger) Error(string, ...interface{}) error {
 	return nil
 }
+
+func (l *nopLogger) With(...interface{}) Logger {
+	return l
+}
+
+func (l *nopLogger) WithLevel(lvl string) Logger {
+	return l
+}