dashboard: add event loop panel and alerts

[DifferentialOrange]

changelog: restore alert example for fiber panels

example_cluster: bump metrics to 0.13.0

Bump metrics to 0.13.0 to test panels for event loop metrics. See metrics CHANGELOG [1] for other updates.

1. https://github.com/tarantool/metrics/blob/master/CHANGELOG.md

dashboard: add event loop panel

alerts: add example warning for event loop time

Follows up #72 Closes #73