Send resolved notification only when all alerts are solved

Similar to #1403, however, our use-case is different

What did you do?

Alert is launched as severity: warning, then raised to severity: critical but its lowered back to severity: warning.

What did you expect to see?

A resolved notification only after the entire (grouped) alarm is solved.

What did you see instead? Under which circumstances?

Critical alarm sends a resolved status. Therefore, the entire alarm is marked as solved.

Environment

Running with the kubernetes-mixin set (https://github.com/kubernetes-monitoring/kubernetes-mixin)

System information:

Kubernetes EKS 1.17

Alertmanager version:

Branch: HEAD
BuildDate: 20190708-14:31:49
BuildUser: root@868685ed3ed0
GoVersion: go1.12.6
Revision: 1ace0f76b7101cccc149d7298022df36039858ca
Version: 0.18.0

Prometheus version:

Version: 2.11.0
Revision: 4ef66003d9855ed2b7a41e987b33828ec36db34d
Branch: HEAD
BuildUser: root@0dc27cf95f36
BuildDate: 20190709-09:54:35
GoVersion: go1.12.7

Alertmanager configuration file:

...
global:
  resolve_timeout: 5m
receivers:
  - name: system-x
    webhook_configs:
      - url: [...]
        send_resolved: true

inhibit_rules:
- source_match: 
    severity: "critical"
  target_match: 
    severity: "warning"
  equal: [ alertname, name, server, common_name ]

route:
  group_by: [cluster, alertname]

  group_wait: 30s
  group_interval: 5m
  repeat_interval: 3h
  receiver: system-x

  routes:
  
  # Alarm for amq monitoring
  - receiver: system-x
    group_wait: 15s
    group_interval: 30s
    match_re:
      job: telegraf-amq-exporter|telegraf-ibm-wmq-exporter
    group_by: [alertname, name]
    repeat_interval: 3m

Timeline of alerts

11:00 -> Alarm 1 with severity: warning
11:00 -> Recieved notification of Alarm 1 with severity: warning
11:05 -> Alarm 1 raised to severity: critical
11:05 -> Recieved notification of Alarm 1 with severity: critical
11:10 -> Alarm 1 lowered to severity: warning
11:10 -> Recieved resolve of Alarm 1 with severity: critical
11:13 -> Recieved notification of Alarm 1 with severity: warning

Expected behaviour
After the entire (grouped) alarm has been cleared, only then the resolve status is being sent.

Start by reproducing the warning-to-critical-to-warning timeline with the provided Alertmanager configuration, focusing on grouped alerts, inhibition, and resolved notifications. Compare the observed critical resolve with the expected behavior, where resolution is sent only after the entire grouped alarm is cleared. No source files or tests are named in the issue.