Few metrics standardization enhancements:
- All probes export the same set of metrics: total/success/latency (replaced sent/rcvd/rtt).
- External probe generated metrics can be CUMULATIVE or GAUGE, based on the config.
Improved GCE instances refresh logic:
- Only use 'RUNNING' instances.
- Introduce a random delay before starting the refresh cycle => If there are multiple cloudprober instances running, all of them will not refresh instance list at the same time.