The timestamp value in my Prometheus alertmanager is not sent to rocket chat webhook

Description:

I send alerts from prometheus to rocketchat webhook. This is the configuration of one of my alert rule:

• alert: HighCpuLoad
  expr: 100 - (avg by (instance, group, host) (irate(node_cpu_seconds_total{mode="idle"}[5m])) * 100) > 81
  for: 5m
  labels:
  severity: warning
  annotations:
  timestamp: "{{ with query "time()" }}{{ . | first | value | humanizeTimestamp }}{{ end }}"
  description: "Check CPU load on host {{ $labels.host }}"
  firing_text: "CPU load on host {{ $labels.host }} is > 80%\n VALUE = {{ humanize $value }}"
  resolved_text: "High CPU load on host {{ $labels.host }} is resolved"

All parameters that I have defined in annotation (description, firing_text and resolved_text) are sent to alertmanager. But I don't receive the "timestamp" in rocketchat alert.

This is the alert in alertmanager UI:

This is the alert message that is appear in rocketchat webhook:

As you can see the timestamp is not shown in alert message.

How can I make it appear in webhook alert messages?

Steps to reproduce:

1. create a webhook channel in rocketchat
2. send alerts from alertmanager to rocketchat webhook

Expected behavior:

The timestamp field should be shown in rocketchat message.

Actual behavior:

Although the timestamp is shown in alertmanager, It is not shown in rocketchat message.

Server Setup Information:

• Version of Rocket.Chat Server:
• Operating System:
• Deployment Method:
• Number of Running Instances:
• DB Replicaset Oplog:
• NodeJS Version:
• MongoDB Version:

Client Setup Information

• Desktop App or Browser Version:
• Operating System:

Additional context

Relevant logs:

No source file or test is named. Start by reproducing the Prometheus Alertmanager webhook flow and trace how alert annotations are converted into the Rocket.Chat message; done means the configured timestamp annotation appears in the delivered alert message, with a regression test covering it.