How should I unit test a method that contains GET calls to external services in Golang

I am coding unit tests in my Go API with Gin Gonic.

Here is my code.

func getKeys(c *gin.Context) {
    var meters []models.Meter

    metadataOperation, err := metadata.GetOperation("AC123456")
    if err != nil {
        sendInternalError(err, c)
        return
    }
    meter, err := metadata.GetMeter("12345")
    // Other instructions
    // ...
    // operation = ...
    c.JSON(http.StatusOK, operation)
}

Here is GetOperation method:

func GetOperation(operationID string) (Operation, error) {
    var operation Operation
    var url = metadataAPIURL + "/v2/operations/" + operationID
    tr := &http.Transport{
        TLSClientConfig: &tls.Config{InsecureSkipVerify: true},
    }
    client := &http.Client{Transport: tr}
    req, err := http.NewRequest("GET", url, nil)

    if err != nil {
        return Operation{}, err
    }
    req.SetBasicAuth(metadataAPIUser, metadataAPIPassword)
    res, err := client.Do(req)
    if err != nil {
        return Operation{}, err
    }
    if res.StatusCode != 200 {
        return Operation{}, errors.New(res.Status)
    }
    err = json.NewDecoder(res.Body).Decode(&operation)
    if err != nil {
        return Operation{}, err
    }
    return operation, nil
}

Thing is metadata.GetOperation("AC123456") will make a GET request to an external service.

As I understand unit testing, I can't have any external dependencies.

In my case, test is passing, but it is making a GET request to my production server which is not the wanted result.

If I want to use mocks, I shoud have an interface, and switch between dependency, and mock.

It should be great to test GetOperation method, but for getKeys method, it seems unclear to me how should I do it.

How should I deal with this situation ? Can anyone give me an example / tuto about this case.

I already checked several tutos about mocking, but I feel it is not responding my use case.