Put the cluster first in an ntfy page title

A phone's lock screen cuts a long title off at the end, and an incident title
keeps its grouping labels there: "PodRestarting (cluster=prod-eu,
namespace=shop)". For a team with a Kubernetes cluster per alert source, the
cluster is the first thing a person wants and the first thing lost.

When the incident has a `cluster` group label, the title of the page now
leads with it, "[prod-eu] PodRestarting (namespace=shop)", and the label is
dropped from the parenthesis so it is not said twice, along with the
parenthesis itself if nothing else is left. The reminder and the resolution
use the same title, and the label is the one the web UI's chip reads. The
message body is unchanged.

An incident without a cluster label gets the same title as before, and
incidents that are already open keep theirs; only pages sent from now on
change. The constant originLabel names the label, as ORIGIN_LABEL does in the
web UI.

No endpoint or JSON shape changed, so nothing to mirror in terdut-tui.

internal/api/notifier.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"log"
 	"net/http"
+	"regexp"
 	"strings"
 	"time"
 
@@ -409,9 +410,11 @@ func renderNotification(inc models.Incident, n outboxRow, firing int, cfg Notify
 			strings.TrimSuffix(cfg.PublicURL, "/"), inc.ID)
 	}
 
+	title := pageTitle(inc)
+
 	switch n.kind {
 	case notifyResolved:
-		msg.Title = "Resolved: " + inc.Title
+		msg.Title = "Resolved: " + title
 		msg.Message = "All alerts stopped firing after " +
 			humanDuration(time.Since(inc.TriggeredAt))
 		msg.Priority = ntfyPriorityLow
@@ -419,9 +422,9 @@ func renderNotification(inc models.Incident, n outboxRow, firing int, cfg Notify
 		return msg
 
 	case notifyReminder:
-		msg.Title = "Still unacknowledged: " + inc.Title
+		msg.Title = "Still unacknowledged: " + title
 	default:
-		msg.Title = inc.Title
+		msg.Title = title
 	}
 
 	severity := derefString(inc.Severity)
@@ -443,6 +446,48 @@ func renderNotification(inc models.Incident, n outboxRow, firing int, cfg Notify
 	return msg
 }
 
+// originLabel is the label that says where an alert came from, for a team with
+// several Kubernetes clusters behind it. It comes from Prometheus's
+// externalLabels and reaches an incident through Alertmanager's group_by; the
+// web UI reads the same label, and the README ("Several clusters, one team")
+// explains how to set it up.
+const originLabel = "cluster"
+
+// pageTitle is the incident's title for a notification. A phone's lock screen
+// cuts a long title off at the end, and the incident title puts the grouping
+// labels there, so the cluster would be the first thing lost. When the incident
+// has an origin it leads instead, "[prod-eu] PodRestarting (namespace=foo)", and
+// is dropped from the parenthesis so it is not said twice. A title that is not
+// in incidentTitle's "name (k=v, k=v)" shape keeps its text and gains the prefix.
+func pageTitle(inc models.Incident) string {
+	origin := inc.GroupLabels[originLabel]
+	if origin == "" {
+		return inc.Title
+	}
+	return "[" + origin + "] " + titleWithoutLabel(inc.Title, originLabel, origin)
+}
+
+var titleShape = regexp.MustCompile(`(?s)^(.*?) \((.*)\)$`)
+
+// titleWithoutLabel removes "key=value" from the parenthesised tail of a title
+// built by incidentTitle, and the parentheses with it if nothing else is left.
+func titleWithoutLabel(title, key, value string) string {
+	m := titleShape.FindStringSubmatch(title)
+	if m == nil {
+		return title
+	}
+	var rest []string
+	for _, part := range strings.Split(m[2], ", ") {
+		if part != key+"="+value {
+			rest = append(rest, part)
+		}
+	}
+	if len(rest) == 0 {
+		return m[1]
+	}
+	return m[1] + " (" + strings.Join(rest, ", ") + ")"
+}
+
 // ntfy's priority scale. Max is the one that overrides the phone's quiet
 // settings, which is the whole point of paging on critical.
 const (