fix monitor data source

[yseto]

Output from acceptance testing:

$ go clean -testcache
$ MACKEREL_EXPERIMENTAL_TFFRAMEWORK=1 make testacc TESTS=TestAccDataSourceMackerelMonitorExternal
TF_ACC=1 go test -v ./... -run TestAccDataSourceMackerelMonitorExternal -timeout 120m
?       github.com/mackerelio-labs/terraform-provider-mackerel  [no test files]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/mackerel        0.384s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/planmodifierutil        0.210s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/provider        0.499s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/typeutil        0.211s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/validatorutil   0.564s [no tests to run]
2025/04/03 23:51:31 [INFO] mackerel: use terraform-plugin-framework based implementation
=== RUN   TestAccDataSourceMackerelMonitorExternal
=== PAUSE TestAccDataSourceMackerelMonitorExternal
=== CONT  TestAccDataSourceMackerelMonitorExternal
--- PASS: TestAccDataSourceMackerelMonitorExternal (2.61s)
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/mackerel 3.504s
$ go clean -testcache
$ MACKEREL_EXPERIMENTAL_TFFRAMEWORK=0 make testacc TESTS=TestAccDataSourceMackerelMonitorExternal
TF_ACC=1 go test -v ./... -run TestAccDataSourceMackerelMonitorExternal -timeout 120m
?       github.com/mackerelio-labs/terraform-provider-mackerel  [no test files]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/mackerel        0.389s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/planmodifierutil        0.203s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/provider        0.612s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/typeutil        0.328s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/validatorutil   0.683s [no tests to run]
=== RUN   TestAccDataSourceMackerelMonitorExternal
=== PAUSE TestAccDataSourceMackerelMonitorExternal
=== CONT  TestAccDataSourceMackerelMonitorExternal
--- PASS: TestAccDataSourceMackerelMonitorExternal (2.18s)
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/mackerel 3.153s
$