victoria-metrics: Send alerts via ntfy

I don't like having alerts sent by e-mail.  Since I don't get e-mail
notifications on my watch, I often do not see alerts for quite some
time.  They are also much harder to read in an e-mail client (Fastmail
web an K-9 Mail both display them poorly).  I would much rather have
them delivered via _ntfy_, just like all the rest of the ephemeral
notifications I receive.

Fortunately, it is easy enough to integrate Alertmanager and _ntfy_
using the webhook notifier in Alertmanager.  Since _ntfy_ does not
natively support the Alertmanager webhook API, though, a bridge is
necessary to translate from one data format to the other.  There are a
few options for this bridge, but I chose
[alexbakker/alertmanager-ntfy][0] because it looked the most complete
while also having the simplest configuration format.  Sadly, it does not
expose any Prometheus metrics itself, and since it's deployed in the
_victoria-metrics_ namespace, it needs to be explicitly excluded from
the VMAgent scrape configuration.

[0]: https://github.com/alexbakker/alertmanager-ntfy

victoria-metrics/alertmanager-ntfy.config.yml

@@ -0,0 +1,24 @@
+http:
+  addr: :8000
+ntfy:
+  baseurl: http://ntfy.ntfy:2586
+  notification:
+    topic: alerts
+    priority: |
+      status == "firing" ? "urgent" : "default"
+    tags:
+    - tag: heavy_check_mark
+      condition: status == "resolved"
+    - tag: rotating_light
+      condition: status == "firing"
+    templates:
+      title: >-
+        {{ if eq .Status "resolved" }}Resolved: {{ end -}}
+        {{ if index .Annotations "summary" -}}
+        {{ index .Annotations "summary" }}
+        {{ else -}}
+        {{ index .Labels "alertname" }}
+        {{ index .Labels "alertgroup" }}
+        {{ end -}}
+      description: |
+        {{ index .Annotations "description" }}