feat!: Change the default value of "monitoring_enable_managed_prometheus" var to null

[legal90]

Fixes #1706

This PR is a continuation of #1715 by removing the condition for monitoring_config and turning it into a "static" block.

This PR is a continuation of #2155.
Here we change the default value of the variable monitoring_enable_managed_prometheus to null.

That is supposed to fix the calculation of local.logmon_config_is_set. Now it will return true not only when monitoring_enable_managed_prometheus is set to true, but also when it is set to false.

[apeabody]

/gcbrun

[apeabody]

Hi @legal90!

From the CI test:

Error: googleapi: Error 400: Cannot specify logging_config or monitoring_config together with logging_service or monitoring_service.
Details:
[
  {
    "@type": "type.googleapis.com/google.rpc.RequestInfo",
    "requestId": "0xbb9f414f03695c18"
  }
]
, badRequest

  with module.example.module.gke.google_container_cluster.primary,
  on ../../../cluster.tf line 22, in resource "google_container_cluster" "primary":
  22: resource "google_container_cluster" "primary" {
}

[legal90]

@apeabody Thank you! I will take a look at it soon.

[legal90]

@apeabody I rolled back my previous changes and implemented the fix using a different approach - by changing the default value of "monitoring_enable_managed_prometheus" var to null.

Could you please re-trigger the integration test?

[apeabody]

/gcbrun

[apeabody]

Thanks for the contribution @legal90!

[apeabody]

/gcbrun