Add prometheus metrics for log4j log entries

Enabled default log4j metrics collector. The number of log4j log entries will be counted. This enable the following measurable metrics (captured after clean startup gitblit):

 # HELP log4j_appender_total Log4j log statements at various log levels
 # TYPE log4j_appender_total counter
 log4j_appender_total{level="debug",} 0.0
 log4j_appender_total{level="warn",} 1.0
 log4j_appender_total{level="trace",} 0.0
 log4j_appender_total{level="error",} 0.0
 log4j_appender_total{level="fatal",} 0.0
 log4j_appender_total{level="info",} 82.0

Author: goettl79

src/main/java/log4j.properties

@@ -17,7 +17,7 @@
 #      FATAL, ERROR, WARN, INFO, DEBUG
 #
 #------------------------------------------------------------------------------
-log4j.rootCategory=INFO, S
+log4j.rootCategory=INFO, S, METRICS
 
 #log4j.rootLogger=INFO
 #log4j.logger.org=INFO
@@ -67,3 +67,6 @@ log4j.appender.H.File = logs/gitblit.html
 log4j.appender.H.MaxFileSize = 100KB
 log4j.appender.H.Append = false
 log4j.appender.H.layout = org.apache.log4j.HTMLLayout
+
+log4j.appender.METRICS = io.prometheus.client.log4j.InstrumentedAppender
+log4j.appender.METRICS.Append = false