Update Azure Spot Termination checking

* Query the Azure metadata service for scheduled termination
events and decode the response JSON. In the past it may have
been sufficient to look for OK/200 responses as the indicator,
but now it seems that the default is a JSON response with zero
events.
* When any scheduled termination event is observed, start the
stop procedures (e.g. drain and/or delete, depending)

Docs: https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-terminate-notification#get-terminate-notifications

Author: dghubble

internal/scuttle.go

@@ -8,6 +8,7 @@ package scuttle
 
 import (
 	"context"
+	"encoding/json"
 	"fmt"
 	"net/http"
 	"os"
@@ -217,6 +218,21 @@ func (w *Scuttle) pendingShutdown(ctx context.Context) bool {
 		return false
 	}
 
+	if w.config.Platform == "azure" {
+		scheduledEvents := new(AzureScheduledEvents)
+		err = json.NewDecoder(resp.Body).Decode(scheduledEvents)
+		if err != nil {
+			w.log.WithFields(fields).Errorf("scuttle: error decoding Azure scheduled events: %v, %v", err, scheduledEvents)
+		}
+		if len(scheduledEvents.Events) > 0 {
+			w.log.WithFields(fields).Info("scuttle: Spot Instance interruption notice!")
+			return true
+		}
+		w.log.WithFields(fields).Debugf("scuttle: no scheduled events: %v", scheduledEvents)
+		return false
+	}
+
+	// by default, a simple 200 response code indicates an interruption notice
 	switch resp.StatusCode {
 	case 200:
 		w.log.WithFields(fields).Info("scuttle: Spot Instance interruption notice!")
@@ -226,3 +242,11 @@ func (w *Scuttle) pendingShutdown(ctx context.Context) bool {
 		return false
 	}
 }
+
+// Github Issue (abbreviated)
+type AzureScheduledEvents struct {
+	DocumentIncarnation int `json:"DocumentIncarnation"`
+	Events              []struct {
+		ID string `json:"EventId"`
+	} `json:"Events"`
+}