Heartbeat check custom fields

[mathieuisabel]

It would be beneficial to support custom fields for individual checks in Heartbeat. For instance, it would be good to be able to have an environment field for the check. Right now we have to stuff this data in the name field which is not convenient for aggregation purposes.

[urso]

TODO: Add support for tags and fields per monitor, as supported by the other beats.

[glefloch]

@urso, I would be interested into contributing on this issue. Is it possible to mark it as adopt me?

[ruflin]

@glefloch no need for the adopt me flag. Just go for it :-D Leave a quick comment here when you start working on it so people know that it's in progress.

[glefloch]

ok, then, I start working on it. :)

[urso]

Any contribution is very welcome ;)

I'd say you want to have a look at the annotated function in util.go.

Note, we're doing some event format update according to #3406 also touching the same function. Just to give you a warning that some merge conflict might come up (easily resolvable via rebase). We can help in PR if this happens.

[glefloch]

Great, thank you.

I will be careful when pushing my changes.