title: AlertManager is not forwarding alerts to webhook receiver
body: <p>We have setup Prometheus in a kubernetes cluster using PrometheusOperator. We are trying to configure AlertManager using the AlertManagerConfig custom resource. We tried creating an alert route which maps to a webhook receiver and then triggering a test alert. The alert seems to be caught by AlertManager but it is not being forwarded to the webhook endpoint. AlertManager pod logs are also not printing any logs regarding notifications being send to the receivers for an alert. Sharing the test config below:</p>
  <pre><code>apiVersion: monitoring.coreos.com/v1alpha1
  kind: AlertmanagerConfig
  metadata:
    name: discord-config
  spec:
    receivers:
    - name: discord
      webhookConfigs:
      - url: '&lt;webhook-url&gt;'
        sendResolved: true
    route:
      groupBy: ['job']
      groupWait: 15s
      groupInterval: 15s
      repeatInterval: 15s
      receiver: 'discord'
  ---
  apiVersion: monitoring.coreos.com/v1
  kind: PrometheusRule
  metadata:
    name: test-rules
  spec:
    groups:
    - name: test-rule-group
      rules:
      - alert: TestAlert
        expr: vector(1)
        labels: 
          severity: medium  
        annotations:
          description: &quot;This is a reciever test for webhook alert&quot;
          summary: &quot;This is a dummy summary&quot;
  </code></pre>
  <p>Is there anything else that needs to be taken care of for the receivers to start receiving alerts?</p>