Added apply_logger_env_levels

[afshinpir]

This method applies levels which is set by environment variable SPDLOG_LEVEL to the a single logger. Useful for loading configuration into manually created loggers.

[tt4g]

Fix #2648

[gabime]

Thanks @afshinpir

[tt4g]

@afshinpir Sorry about this after merged. I forgot to submit my review.
Could you please open PR to remove std::lock_guard<std::mutex> lock(logger_map_mutex_); in registry::apply_logger_env_levels()?
This is because logger_map_mutex_ is std::mutex to protect loggers_ variables.

spdlog/include/spdlog/details/registry-inl.h

Lines 290 to 296 in 51bcff8

	SPDLOG_INLINE void registry::apply_logger_env_levels(std::shared_ptr<logger> new_logger)
	{
	std::lock_guard<std::mutex> lock(logger_map_mutex_);
	auto it = log_levels_.find(new_logger->name());
	auto new_level = it != log_levels_.end() ? it->second : global_log_level_;
	new_logger->set_level(new_level);
	}

[afshinpir]

@tt4g I can open a new PR, but since log_levels_ is a map, so we may need to have excusive access to it for thread safety. Since registry::set_levels and registry::initialize_logger both use this mutex, I used the same one. So this mutex currently protects access to both log_levels_ and loggers_. I think it is better to have it like this rather than adding new critical section. or I can just remove it and assume that set_levels and apply_logger_env_levels will never be called at same time. Please let me know what to do.

[tt4g]

@afshinpir I agree with your thoughts.
Because of the variable names I was unaware that logger_map_mutex_ protected loggers_ and log_levels_.
There would be no need to open a PR.