Prometheus AlertManager alerts not being forwarded to Slack channels

Prometheus server
1

Prometheus ver. 2.47.0 / AlertManager ver. 0.26.0

I’m trying to route AlertManager alerts to 2 different Slack channels.

Prometheus config file

  - job_name: 'node_exporter_metrics_staging'
    scrape_interval: 5s
    static_configs:
      - targets:
          [
            'staging.domain.tld:9100',
          ]
        labels:
          environment: "staging"


  - job_name: 'node_exporter_metrics_test'
    scrape_interval: 5s
    static_configs:
      - targets:
          [
            'test.cli:9100',
          ]
        labels:
          environment: "test"

AlertManager config file

route:
  group_by: ['alertname', 'group', 'instance']
  group_wait: 30s
  group_interval: 5m
  repeat_interval: 1h
  # default receiver
  receiver: 'test-receiver'
  routes:
    - matchers:
        - environment="staging"
      receiver: staging-receiver
      continue: true
    - matchers:
        - environment="test"
      receiver: test-receiver
      continue: true

receivers:

  # STAGING RECEIVER
  - name: 'staging-receiver'
    slack_configs:
    - api_url: https://xxx
      channel: "#devops-alerts-staging"
      send_resolved: true
      text: >-
        {{ range .Alerts -}}
        *Alert:* {{ .Annotations.title }}{{ if .Labels.severity }} - `{{ .Labels.severity }}`{{ end }}
        *Description:* {{ .Annotations.description }}
        *Details:*
          {{ range .Labels.SortedPairs }} • *{{ .Name }}:* `{{ .Value }}`
          {{ end }}
        {{ end }}


 # TEST RECEIVER
  - name: 'test-receiver'
    slack_configs:
    - api_url: https://xxx
      channel: "#devops-alerts-test"
      send_resolved: true
      text: >-
        {{ range .Alerts -}}
        *Alert:* {{ .Annotations.title }}{{ if .Labels.severity }} - `{{ .Labels.severity }}`{{ end }}
        *Description:* {{ .Annotations.description }}
        *Details:*
          {{ range .Labels.SortedPairs }} • *{{ .Name }}:* `{{ .Value }}`
          {{ end }}
        {{ end }}

Alert rules

groups:
- name: alert.rules
  rules:
  # INSTANCE DOWN
  - alert: Instance_Down
    expr: up == 0
    for: 1m
    labels:
      severity: "critical"
    annotations:
      summary: "Instance {{ $labels.instance }} is down"
      description: "{{ $labels.instance }} has been down for more than 1 minutes."

  # HIGH CPU LOAD
  - alert: Instance_High_Cpu_Load
    expr: 100 - (avg by(instance) (rate(node_cpu_seconds_total{mode="idle"}[2m])) * 100) > 90
    for: 0m
    labels:
      severity: warning
    annotations:
      summary: Instance high CPU load (instance {{ $labels.instance }})
      description: CPU load is > 90%. VALUE = {{ $value }}

  # INSTANCE OUT OF MEMORY
  - alert: Instance_Out_Of_Memory
    expr: node_memory_MemAvailable_bytes / node_memory_MemTotal_bytes * 100 < 20
    for: 2m
    labels:
      severity: warning
    annotations:
      summary: Instance out of memory (instance {{ $labels.instance }})
      description: Instance RAM memory is filling up (< 20% left). VALUE = {{ $value }}

  # INSTANCE OUT OF DISK SPACE
  - alert: Instance_Out_Of_Disk_Space
    expr: (node_filesystem_avail_bytes * 100) / node_filesystem_size_bytes < 10 and ON (instance, device, mountpoint) node_filesystem_readonly == 0
    for: 2m
    labels:
      severity: warning
    annotations:
      summary: Instance out of disk space (instance {{ $labels.instance }})
      description: Disk is almost full (< 10% left). VALUE = {{ $value }}

The problem is that alerts are being forwarded only to default receiver (test-receiver).
I’m not sure about the matchers syntax, I setup labels in Prometheus config file and I would like to forward alerts following the environment type.
I’ve been looking up for some examples, but info is often misleading.
Nothing relevant in syslog, and in official docs.

Which is the best way to setup this?