Send alert to slack from a Prometheus helm chart - kubernetes

Question

I am trying to create alerts in Prometheus on Kubernetes and sending them to a Slack channel. For this i am using the prometheus-community helm-charts (which already includes the alertmanager). As i want to use my own alerts I have also created an values.yml (shown below) strongly inspired from here.
If I port forward Prometheus I can see my Alert there going from inactive, to pending to firing, but no message is sent to slack. I am quite confident that my alertmanager configuration is fine (as I have tested it with some prebuild alerts of another chart and they were sent to slack). So my best guess is that I add the alert in the wrong way (in the serverFiles part), but I can not figure out how to do it correctly. Also, the alertmanager logs look pretty normal to me. Does anyone have an idea where my problem comes from?
---
serverFiles:
alerting_rules.yml:
groups:
- name: example
rules:
- alert: HighRequestLatency
expr: sum(rate(container_network_receive_bytes_total{namespace="kube-logging"}[5m]))>20000
for: 1m
labels:
severity: page
annotations:
summary: High request latency
alertmanager:
persistentVolume:
storageClass: default-hdd-retain
## Deploy alertmanager
##
enabled: true
## Service account for Alertmanager to use.
## ref: https://kubernetes.io/docs/tasks/configure-pod-container/configure-service-account/
##
serviceAccount:
create: true
name: ""
## Configure pod disruption budgets for Alertmanager
## ref: https://kubernetes.io/docs/tasks/run-application/configure-pdb/#specifying-a-poddisruptionbudget
## This configuration is immutable once created and will require the PDB to be deleted to be changed
## https://github.com/kubernetes/kubernetes/issues/45398
##
podDisruptionBudget:
enabled: false
minAvailable: 1
maxUnavailable: ""
## Alertmanager configuration directives
## ref: https://prometheus.io/docs/alerting/configuration/#configuration-file
## https://prometheus.io/webtools/alerting/routing-tree-editor/
##
config:
global:
resolve_timeout: 5m
slack_api_url: "I changed this url for the stack overflow question"
route:
group_by: ['job']
group_wait: 30s
group_interval: 5m
repeat_interval: 12h
#receiver: 'slack'
routes:
- match:
alertname: DeadMansSwitch
receiver: 'null'
- match:
receiver: 'slack'
continue: true
receivers:
- name: 'null'
- name: 'slack'
slack_configs:
- channel: 'alerts'
send_resolved: false
title: '[{{ .Status | toUpper }}{{ if eq .Status "firing" }}:{{ .Alerts.Firing | len }}{{ end }}] Monitoring Event Notification'
text: >-
{{ range .Alerts }}
*Alert:* {{ .Annotations.summary }} - `{{ .Labels.severity }}`
*Description:* {{ .Annotations.description }}
*Graph:* <{{ .GeneratorURL }}|:chart_with_upwards_trend:> *Runbook:* <{{ .Annotations.runbook }}|:spiral_note_pad:>
*Details:*
{{ range .Labels.SortedPairs }} • *{{ .Name }}:* `{{ .Value }}`
{{ end }}
{{ end }}

Answer 1

So I have finally solved the problem. The problem apparently was that the kube-prometheus-stack and the prometheus helm charts work a bit differently.
So instead of alertmanager.config I had to insert the code (everything starting from global) at alertmanagerFiles.alertmanager.yml.