[Heartbeat] Support for random/range monitor interval

[gaby]

Describe the enhancement:
The current schedule property in Heartbeat only allows for fixed intervals. It would be useful to allow for random intervals or ranges. For example "Send ICMP between 30 to 60 secs".

Describe a specific use case for the enhancement or feature:

• Heartbeats are not only used for uptime checks of internal services, they can also be used for external services. Sending consistent checks to external services is prune to being blocked or rate-limited.
• If i want to check internet connectivity to lets say google.com, I don't want to ping them every 5 secs. I rather ping them between 30 to 60 seconds which would give me an uptime on a per-minute basis.

[botelastic]

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