[doc][log] Add survey for glog #26

- it has interesting features like LOG_IF, LOG_EVERY
- most stuff are squished into one `glog.go` file
- the cpp version has more features, and the code is a little bit hard
to read due to using automake to generate header file (h.in)

log2/doc/glog.md

@@ -1,3 +1,50 @@
 # glog
 
-https://github.com/golang/glog
+https://github.com/golang/glog
+
+- supports rotate file and log to different file?
+- log methods like `Info`, `Infof` are implemented on `Verbose` which is a boolean indicate if required level should be logged
+- includes buffer etc.
+- lock when log
+- it **collects counter** (needed for fancy features like when logging hits line file:N emit a stack trace)
+
+````go
+if glog.V(2) {
+    glog.Info("Starting transaction...")
+}
+
+glog.V(2).Infoln("Processed", nItems, "elements")
+````
+
+````go
+func (l *loggingT) output(s severity, buf *buffer, file string, line int, alsoToStderr bool) {
+	l.mu.Lock()
+	// ...
+    l.mu.Unlock()
+    if stats := severityStats[s]; stats != nil {
+        atomic.AddInt64(&stats.lines, 1)
+        atomic.AddInt64(&stats.bytes, int64(len(data)))
+    }
+}
+````
+
+flags, including interesting features
+
+````go
+func init() {
+	flag.BoolVar(&logging.toStderr, "logtostderr", false, "log to standard error instead of files")
+    flag.BoolVar(&logging.alsoToStderr, "alsologtostderr", false, "log to standard error as well as files")
+    flag.Var(&logging.verbosity, "v", "log level for V logs")
+    flag.Var(&logging.stderrThreshold, "stderrthreshold", "logs at or above this threshold go to stderr")
+    flag.Var(&logging.vmodule, "vmodule", "comma-separated list of pattern=N settings for file-filtered logging")
+    flag.Var(&logging.traceLocation, "log_backtrace_at", "when logging hits line file:N, emit a stack trace")
+}
+````
+
+The cpp version actually supports more features 
+
+https://github.com/google/glog
+
+`LOG_IF(INFO, num_cookies > 10)`, `LOG_EVERY_N(INFO, 10)`, , `LOG_IF_EVERY_N`, `LOG_FIRST_N(INFO, 20)`
+
+- macro is used to expend those to real function calls, it is using automake `logging.h.in` so