hosts: Add mtrcs0.p.r

*mtrcs0.pyrocufflink.red* is a Raspberry Pi CM4 on a Waveshare
CM4-IO-BASE-B carrier board with a NVMe SSD.  It runs a custom OS built
using Buildroot, and is not a member of the *pyrocufflink.blue* AD
domain.

*mtrcs0.p.r* hosts Victoria Metrics/`vmagent`, `vmalert`, AlertManager,
and Grafana.  I've created a unique group and playbook for it,
*metricspi*, to manage all these applications together.

group_vars/metricspi/alertmanager.yml

@@ -0,0 +1,24 @@
+alertmanager_config:
+  global:
+    smtp_from: prometheus@pyrocufflink.blue
+    smtp_smarthost: mail.pyrocufflink.blue:25
+    smtp_require_tls: false
+
+  route:
+    receiver: default-email
+    group_by:
+    - ...
+    routes:
+    - receiver: default-email
+      group_wait: 1m
+      group_by:
+      - alertname
+      repeat_interval: 120h
+      match:
+        job: homeassistant
+
+  receivers:
+  - name: default-email
+    email_configs:
+    - to: gyrfalcon@ebonfire.com
+      send_resolved: true