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.config.yml

@@ -8,16 +8,11 @@ receivers:
   - send_resolved: true
     to: gyrfalcon@ebonfire.com
   name: default-email
+- name: ntfy
+  webhook_configs:
+  - url: http://alertmanager-ntfy:8000/hook
 
 route:
   group_by:
   - '...'
-  receiver: default-email
-  routes:
-  - group_by:
-    - alertname
-    group_wait: 1m
-    match:
-      job: homeassistant
-    receiver: default-email
-    repeat_interval: 120h
+  receiver: ntfy