Makes SubstituteLogger location aware

[ppkarwasz]

Currently, SubstituteLogger neither supports location information from upstream callers nor passes location information to the delegated logger.

This PR:

• implements LocationAwareLogger, which allows upstream callers to pass location information both through the LocationAwareLogger API and the fluent API.
• sets the correct caller boundary, when the classic API methods are invoked.

This is related to #425.

[ceki]

@ppkarwasz Thank you for this PR.

For the moment, I fail to see the circumstances under which the LocationAwareLogger.log() method may be called. Still investigating though.

[ppkarwasz]

For the moment, I fail to see the circumstances under which the LocationAwareLogger.log() method may be called. Still investigating though.

I am not entirely sure when SubstituteLogger is used (or if it is used at all these days), but I assume that it could appear in OSGi environments if:

1. The SLF4J bundle is activated first (or used by another bundle without being activated).
2. The Logback bundle is activated only later.

Since LocationAwareLogger is the oldest available mechanism to transfer caller boundary information, it is commonly used in API-to-API bridges, e.g. the Log4j API-to-SLF4J bridge or Spring JCL-to-SLF4J bridge.

Newer mechanisms such as LoggingEventAware (for logger implementations) or CallerBoundaryAware (for logger builder implementations) are less common. The default LoggingEventBuilder also requires the underlying logger to either implement LocationAwareLogger or LoggingEventAware:

slf4j/slf4j-api/src/main/java/org/slf4j/spi/DefaultLoggingEventBuilder.java

Lines 157 to 169 in 69c333d

	protected void log(LoggingEvent aLoggingEvent) {
	if(aLoggingEvent.getCallerBoundary() == null) {
	setCallerBoundary(DLEB_FQCN);
	}
	if(logger instanceof LoggingEventAware) {
	((LoggingEventAware) logger).log(aLoggingEvent);
	} else if(logger instanceof LocationAwareLogger) {
	logViaLocationAwareLoggerAPI((LocationAwareLogger) logger, aLoggingEvent);
	} else {
	logViaPublicSLF4JLoggerAPI(aLoggingEvent);
	}
	}