Add 'unknown' monitor threshold field

[visciang]

This PR adds support for Datadog monitor threshold field.

Giovanni

[ojongerius]

LGTM! I am wondering what the unknown threshold does. I can't find a reference to it in regards to monitors at https://docs.datadoghq.com/api/?lang=bash#get-a-monitor-s-details @visciang what is it supposed to provide?

[entropiae]

It looks like it isn't in the documentation (we are opening a PR for the DD API documentation); it set the threshold after which a service check in unknown status trigger a monitor; it's the grey slider in the attached screenshot.
Here there is the documentation service-check side: https://docs.datadoghq.com/agent/agent_checks/#sending-service-checks

[ojongerius]

Thanks for the work and feedback!

[entropiae]

Thank you too 😃

[visciang]

@ojongerius please, could you tag a new release, so that I can reference it in the PR I'm opening to terraform-providers/terraform-provider-datadog?

Thank you

[ghost]

@visciang I tagged this commit as v2.8