Add new monitor for Labeled Metric

[mashiike]

In the current version, an error occurs when running the code below.

package main

import (
	"errors"
	"log"
	"os"

	"github.com/mackerelio/mackerel-client-go"
)

func main() {
	client := mackerel.NewClient(os.Getenv("MACKEREL_APIKEY"))
	monitor, err := client.GetMonitor(os.Getenv("MONITOR_ID")) // query monitor
	if err != nil {
		var apiError *mackerel.APIError
		if errors.As(err, &apiError) {
			log.Fatalf("API error: %#v", apiError)
		}
		log.Fatalf("%#v", err)
	}
	log.Printf("monitor: %v", monitor)
}

$ go run main.go 
2024/04/02 11:28:56 &mackerel.unknownMonitorTypeError{Type:"query"}
exit status 1

When I checked the API with Curl as shown below, it seemed to be able to be obtained successfully.

$  curl -H "X-Api-Key:$MACKEREL_APIKEY" https://api.mackerelio.com/api/v0/monitors/<monitor_id>
{"monitor":{"critical":300,"isMute":false,"legend":"","query":"test.my_counter","name":"my counter","warning":30,"id":"<monitor_id>","type":"query","operator":">"}}%     

Therefore, in this PR, we modified it to be compatible with query monitor.
By applying this PR, the execution result of the code at the beginning will be as follows.

$ go run main.go 
2024/04/02 11:27:47 monitor: &{<monitor_id> my counter  query false 0 test.my_counter > 0x1400018c4d0 0x1400018c4a8 }

[lufia]

Thank you!