Add a metric for tracking heap size after last gc event

[lenin-jaganathan]

Please describe the feature request. In most cases for performance optimizations related to memory, it is necessary to understand the GC behavior and the memory allocation/reclamation behaviors. It is in this aspect, that analyzing the various heap region sizes after GC seems to be a very useful metric. The information can be obtained from MemoryPoolMXBean#getCollectionUsage on supported JVM's.

Related metrics in this space, but having the exact snapshot data post-GC helps in applying transformations/aggregations in the backend along with the memory pool names.

• jvm.gc.memory.promoted
• jvm.gc.memory.allocated
• jvm.gc.max.data.size