Per bucket monitor, metric values get in email alert with mustache -> how?

Hi there,
Have a “per bucket monitor” active here.
The values of “group by”, in this case the hostnames are displayed correctly.
I also want to display the metrics values matching the hostname. How do i do that here ?

I would like to extract the value “avg_cpu_usage_user”, how do i do that ?
query response:

    "aggregations": {
        "composite_agg": {
            "buckets": [
                {
                    "doc_count": 974,
                    "key": {
                        "tags.host": "SFG24"
                    },
                    "avg_cpu_usage_user": {
                        "value": 5.083928505579631
                    }
                },
                {
                    "doc_count": 150,
                    "key": {
                        "tags.host": "ingest01"
                    },
                    "avg_cpu_usage_user": {
                        "value": 60.62625534057617
                    }
                },
                {
                    "doc_count": 180,
                    "key": {
                        "tags.host": "os-dash01"
                    },
                    "avg_cpu_usage_user": {
                        "value": 0.3969384260475636
                    }
                }
            ],
            "after_key": {
                "tags.host": "os-dash01"
            }
        }
    }
}

Email-Template:

Monitor {{ctx.monitor.name}} just entered alert status. Please investigate the issue.
  - Trigger: {{ctx.trigger.name}}
  - Severity: {{ctx.trigger.severity}}
  - Period start: {{ctx.periodStart}}
  - Period end: {{ctx.periodEnd}}

  - Deduped Alerts:
  {{#ctx.dedupedAlerts}}
    * {{id}} : {{bucket_keys}}  {{avg_cpu_usage_user.value}}
  {{/ctx.dedupedAlerts}}

  - New Alerts:
  {{#ctx.newAlerts}}
    * {{id}} : {{bucket_keys}}  {{avg_cpu_usage_user.value}}
  {{/ctx.newAlerts}}

  - Completed Alerts:
  {{#ctx.completedAlerts}}
    * {{id}} : {{bucket_keys}} {{avg_cpu_usage_user.value}}
  {{/ctx.completedAlerts}}