Implement canary external check

- do a HTTP POST for each webhook registered in the canary analysis
- increment the failed checks counter if a webhook returns a non-2xx status code and log the error and the response body if exists

pkg/controller/webhook.go

@@ -0,0 +1,71 @@
+package controller
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	flaggerv1 "github.com/stefanprodan/flagger/pkg/apis/flagger/v1alpha2"
+	"io/ioutil"
+	"net/http"
+	"net/url"
+	"time"
+)
+
+// CallWebhook does a HTTP POST to an external service and
+// returns an error if the response status code is non-2xx
+func CallWebhook(name string, namepace string, w flaggerv1.CanaryWebhook) error {
+
+	payload := flaggerv1.CanaryWebhookPayload{
+		Name:      name,
+		Namespace: namepace,
+		Metadata:  w.Metadata,
+	}
+
+	payloadBin, err := json.Marshal(payload)
+	if err != nil {
+		return err
+	}
+
+	hook, err := url.Parse(w.URL)
+	if err != nil {
+		return err
+	}
+
+	req, err := http.NewRequest("POST", hook.String(), bytes.NewBuffer(payloadBin))
+	if err != nil {
+		return err
+	}
+
+	req.Header.Set("Content-Type", "application/json")
+
+	if len(w.Timeout) < 2 {
+		w.Timeout = "10s"
+	}
+
+	timeout, err := time.ParseDuration(w.Timeout)
+	if err != nil {
+		return err
+	}
+
+	ctx, cancel := context.WithTimeout(req.Context(), timeout)
+	defer cancel()
+
+	r, err := http.DefaultClient.Do(req.WithContext(ctx))
+	if err != nil {
+		return err
+	}
+	defer r.Body.Close()
+
+	b, err := ioutil.ReadAll(r.Body)
+	if err != nil {
+		return fmt.Errorf("error reading body: %s", err.Error())
+	}
+
+	if r.StatusCode > 202 {
+		return errors.New(string(b))
+	}
+
+	return nil
+}