Feature Request: Sort Alerts based on startsAt time or arbitrary paramater

What did you do?
We are using a template to render slack alerts for Alertmanager (using a range .Aletrts). However, when grouping alerts it would be helpful to show the most recent alert first - especially to easily identify if an alert is flapping and/or determine quickly what alert has most recently fired in a particular group.

What did you expect to see?
It would be convenient to have a method to sort alerts based on a particular label or parameter in .Alerts.

Something like a template function (sortAsc or sortDesc, either lexicographically for strings or just in numerical order for numbers.

• Example template:

{{/* The test to display in the alert */}}
{{ define "slack.alerts.text" -}}
    {{ range .Alerts }}
        {{- if .Annotations.summary }}
*Summary: * {{ .Annotations.summary }}
        {{- end }}
        {{- if .GeneratorURL }}
:chart_with_upwards_trend: *<{{ .GeneratorURL }}|Graph>*
        {{- end }}
        {{- if .Annotations.runbook_url }}
:spiral_note_pad: *<{{ .Annotations.runbook_url }}|Runbook URL>*
        {{- end }}
        {{- if .Annotations.observability_url }}
:eyes: *<{{ .Annotations.observability_url }}|Observability URL>*
        {{- end }}
        {{- if .Annotations.slack_contact }}
:person_in_tuxedo: *Owner:* <@{{ .Annotations.slack_contact }}> 
        {{- end }}
        {{- if .Annotations.description }}
*Description: * {{ .Annotations.description }}
        {{- end }}
    {{- end }}
{{- end }}

Start with the Alertmanager template handling behind the .Alerts value and review how template functions are exposed. Define the supported sorting behavior for alert fields or parameters, then verify the example Slack template renders alerts in the requested order.