JDK-6858051 added better logging of GC worker creation at a trace level using -Xlog:gc+task=trace

Such information would be very helpful to diagnose startup/warmup behavior, especially when doing performance work on GC ergonomics. However, the current output doesn't give much information about the type of worker created, and it's not clear the correct (per-worker-category) active_workers value is printed.

Example output:

java -Xlog:gc+task=trace -version
[0.016s][trace][gc,task] AdaptiveSizePolicy::add_workers() : active_workers: 23 created_workers: 23
[0.023s][trace][gc,task] AdaptiveSizePolicy::add_workers() : active_workers: 23 created_workers: 6
java version "9-internal"
Java(TM) SE Runtime Environment (build 9-internal+0-2016-05-18-133326.clredest.dev)
Java HotSpot(TM) 64-Bit Server VM (build 9-internal+0-2016-05-18-133326.clredest.dev, mixed mode)