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.

metricspi.yml

@@ -0,0 +1,16 @@
+- hosts: metricspi
+  roles:
+  - role: trustca
+    ca: dch-root-ca
+    # The metricspi OS does not include tools for managing the system
+    # CA trust store.  Fortunately, the only application that needs to
+    # use a custom CA is Grafana, which supports using an alternate CA
+    # certificate path.
+    ca_store_dir: /etc/ssl/certs
+    ca_update_cmd: 'true'
+    tags:
+    - trustca
+
+- import_playbook: victoria-metrics.yml
+- import_playbook: alertmanager.yml
+- import_playbook: grafana.yml