Default request headers for monitoring are inconsistent with the API and Web console

[tosuke]

The default HTTP request header for external monitoring is Cache-Control: no-cache in the API and Web console.
But in the terraform provider, no header is specified. It is inconsistent and seems not to be helpful.

It is possible to match the behavior by specifying Cache-Control: no-cache as the default, but this is a breaking change.

terraform-provider-mackerel/mackerel/resource_mackerel_monitor.go

Lines 259 to 264 in 9a863e6

	"headers": {
	Type: schema.TypeMap,
	Optional: true,
	Sensitive: true,
	Elem: &schema.Schema{Type: schema.TypeString},
	},