Adding help information to log tags

[Ioi Lam]

Now there are a lot of log tags [1], and it looks like many more will be 
added. I start to wonder what 'ihop' or 'rt' would do ...

Would it make sense to add help information that can be printed to the 
user, like

#define LOG_TAG_LIST \
   LOG_TAG(alloc,      "this prints something about alloc") \
   LOG_TAG(age,        "blah blah") \

$ java -Xlog:help=alloc
alloc - this prints something about alloc

$ java -Xlog:help=all
alloc - this prints something about alloc
age   - blah blah


-----------------

[1] The list of log tags as of today in jdk9/hs-rt/hotspot

#define LOG_TAG_LIST \
   LOG_TAG(alloc) \
   LOG_TAG(age) \
   LOG_TAG(barrier) \
   LOG_TAG(bot) \
   LOG_TAG(census) \
   LOG_TAG(classhisto) \
   LOG_TAG(classinit) \
   LOG_TAG(comp) \
   LOG_TAG(compaction) \
   LOG_TAG(cpu) \
   LOG_TAG(cset) \
   LOG_TAG(defaultmethods) \
   LOG_TAG(ergo) \
   LOG_TAG(exceptions) \
   LOG_TAG(exit) \
   LOG_TAG(freelist) \
   LOG_TAG(gc) \
   LOG_TAG(heap) \
   LOG_TAG(humongous) \
   LOG_TAG(ihop) \
   LOG_TAG(jni) \
   LOG_TAG(liveness) \
   LOG_TAG(logging) \
   LOG_TAG(marking) \
   LOG_TAG(metaspace) \
   LOG_TAG(monitorinflation) \
   LOG_TAG(phases) \
   LOG_TAG(plab) \
   LOG_TAG(promotion) \
   LOG_TAG(ref) \
   LOG_TAG(refine) \
   LOG_TAG(region) \
   LOG_TAG(remset) \
   LOG_TAG(rt) \
   LOG_TAG(safepoint) \
   LOG_TAG(scavenge) \
   LOG_TAG(scrub) \
   LOG_TAG(start) \
   LOG_TAG(state) \
   LOG_TAG(stats) \
   LOG_TAG(stringdedup) \
   LOG_TAG(survivor) \
   LOG_TAG(svc) \
   LOG_TAG(sweep) \
   LOG_TAG(task) \
   LOG_TAG(tlab) \
   LOG_TAG(time) \
   LOG_TAG(verify) \
   LOG_TAG(vmoperation)