[ZOOKEEPER-3856] Add a couple metrics to track inflight diff syncs and snap syncs

• Type: Sub-task
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 3.7.0, 3.6.2
• Component/s: metric system
• Assignee: Jie Huang
• Reporter: Jie Huang
• Created: 07/Jun/20 18:06
• Updated: 10/Sep/20 10:43
• Resolved: 08/Jun/20 12:48

Description

There are useful metrics in a cluster with a large number of observes:

INFLIGHT_SNAP_COUNT

INFLIGHT_DIFF_COUNT