More metrics about PageStorage

[JaySon-Huang]

Enhancement

• The latency of reading pages from PageStorage, and some breakdown:
  • latency of getting PageEntry from PageDirectory
  • latency of reading PageData from BlobStore
• The latency of writing pages into PageStorage, and some breakdown:
  • latency of getting insert offset by SpaceMap
  • latency of persisting PageData into BlobFile
  • latency of applying changes into PageDirectory
• Breakdown the duration time of PageStorage GC
• The number of pages in memory
• The number of log files on disk
• The total size of log files on disk
• The memory usage when running GC

[flowbehappy]

Another related metric: the avg and max column files count per segment in the delta layer.