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/kustomization.yaml

@@ -20,6 +20,7 @@ resources:
 - vmagent.yaml
 - vmalert.yaml
 - alertmanager.yaml
+- alertmanager-ntfy.yaml
 - blackbox-exporter.yaml
 - ingress.yaml
 - ../dch-root-ca
@@ -43,6 +44,10 @@ configMapGenerator:
   options:
     disableNameSuffixHash: true
 
+- name: alertmanager-ntfy
+  files:
+  - config.yml=alertmanager-ntfy.config.yml
+
 - name: blackbox
   files:
   - blackbox.yml