Logging: review once again

[antkmsft]

• Default log level, what it should be - Verbose, or Warning, or something else?
• Do we like free functions inside the Azure::Core::Logging:: namespace, or should we make a class?
• std::function, or something else for callback?
• Add string componentId? (other proposal was string serviceId) Or don't? Or not a string, but some enum?
• LogLevels order - ascending or descending? - https://docs.microsoft.com/en-us/dotnet/api/microsoft.extensions.logging.loglevel?view=dotnet-plat-ext-5.0
• Do we want more log levels? Any problems if we add more in the future?

Victor had cool proposal for the LoggingOptions, so logging callback+logLevel would behave something like HttpTransport - you don't have to specify it, or use different transport, but if you want, it is overridable.

• Go and instrument more code with logging? One person does it, or everyone does their part? Storage does not currently have logging at all. Let's also come up with pattern, which log levels we use. Create a separate work item for that?

• Use atomic instead of mutex? I'm pretty sure no one would disagree, but what if would require us to not use std::function in the user-facing API? (Logging: use atomics instead of mutex #1638)

(Just in case: #1568)

[ahsonkhan]

• std::function, or something else for callback?

@CaseyCarter, I would love to hear your perspective on this. One of our goals with logging within the SDK is simplicity. We have quite a straightforward API set and model, which relies on the customer defining this callback method and registering it.
Is using std::function the best (and simplest or the customer) for such a callback?

azure-sdk-for-cpp/sdk/core/azure-core/inc/azure/core/logging/logging.hpp

Lines 41 to 57 in 2d4d766

	typedef std::function<void(LogLevel level, std::string const& message)> LogListener;
	/**
	* @brief Set the function that will be invoked to report an SDK log message.
	*
	* @param logListener A #LogListener function that will be invoked when the SDK reports a log
	* message matching one of the log levels passed to #SetLogLevel(). If `nullptr`, no
	* function will be invoked.
	*/
	void SetLogListener(LogListener logListener);
	/**
	* @brief Sets the #LogLevel an application is interested in receiving.
	*
	* @param level Maximum log level.
	*/
	void SetLogLevel(LogLevel level);