Logger: Make the logger more const friendly.

This change fixes no issues but it does enable the fixing of several
issues and should eliminate issues going forward.

Signed-off-by: Charlie Vigue <charlie.vigue@openvpn.com>

openvpn/log/logger.hpp

@@ -87,7 +87,7 @@ class Logger
  * @param msg the message to print
  */
  template <int LEVEL, typename T>
- void log(T &&msg)
+ void log(T &&msg) const
  {
  /* this ensures that the function is empty if MAX_LEVEL excludes this level */
  if constexpr (max_log_level >= LEVEL)
@@ -103,7 +103,7 @@ class Logger
  * @param msg the message to print
  */
  template <typename T>
- void log_trace(T &&msg)
+ void log_trace(T &&msg) const
  {
  log<LOG_LEVEL_TRACE>(std::forward<T>(msg));
  }
@@ -114,7 +114,7 @@ class Logger
  * @param msg the message to print
  */
  template <typename T>
- void log_debug(T &&msg)
+ void log_debug(T &&msg) const
  {
  log<LOG_LEVEL_DEBUG>(std::forward<T>(msg));
  }
@@ -125,7 +125,7 @@ class Logger
  * @param msg the message to print
  */
  template <typename T>
- void log_info(T &&msg)
+ void log_info(T &&msg) const
  {
  log<LOG_LEVEL_INFO>(std::forward<T>(msg));
  }
@@ -135,7 +135,7 @@ class Logger
  * @param msg the message to log
  */
  template <typename T>
- void log_verbose(T &&msg)
+ void log_verbose(T &&msg) const
  {
  log<LOG_LEVEL_VERB>(std::forward<T>(msg));
  }
@@ -145,7 +145,7 @@ class Logger
  * @param msg the message to log
  */
  template <typename T>
- void log_error(T &&msg)
+ void log_error(T &&msg) const
  {
  log<LOG_LEVEL_ERROR>(std::forward<T>(msg));
  }