Add Period and uptime to Heartbeat Events

[andrewvc]

Add period to heartbeat events, similar to #6929 . We should also add an uptime metric allowing us to use derivatives to track amount of time up/down.

This is necessary for calculating 'no data' states in timelines.

See #12831 an experimental version of this that went with the experimental timeline demo.

[elasticmachine]

Pinging @elastic/uptime

[andrewvc]

After a discussion with @ruflin we're going to defer this. It seems like we can do this better by having job definitions instead of doing this per event.

[andrewvc]

Closing this because we should orient the issue around a use case. We can always make an implementation issue later if needed.