Refeactor the Topology Events and Intent Events Metrics modules:
 * Use the new class EventMetric to cleanup and simplify the implementation
 * Replaced the single metric for Topology Events with four metrics
   (last event timestamp and event rate):
   - Device Event metrics
   - Host Event metrics
   - Link Event metrics
   - Topology Graph metrics

Change-Id: I2acc06ab84ef3ca06d0d383983dfcff9774ff341
6 files changed