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.

hosts

@@ -1,12 +1,18 @@
 [all:vars]
 ansible_python_interpreter=/usr/bin/python3
 
+[alertmanager:children]
+metricspi
+
 [aria2]
 file0.pyrocufflink.blue
 
 [bitwarden_rs]
 bw0.pyrocufflink.blue
 
+[blackbox-exporter:children]
+metricspi
+
 [burp-client]
 bw0.pyrocufflink.blue
 cloud0.pyrocufflink.blue
@@ -18,6 +24,9 @@ burp1.pyrocufflink.blue
 
 [certbot]
 
+[collectd]
+mtrcs0.pyrocufflink.red
+
 [collectd:children]
 pyrocufflink
 
@@ -53,6 +62,7 @@ nvr1.pyrocufflink.blue
 git0.pyrocufflink.blue
 
 [grafana]
+mtrcs0.pyrocufflink.red
 stats0.pyrocufflink.blue
 
 [graylog]
@@ -78,6 +88,9 @@ k8s-amd64-n2.pyrocufflink.blue
 k8s-controller
 k8s-node
 
+[metricspi]
+mtrcs0.pyrocufflink.red
+
 [motioneye]
 
 [named-server:children]
@@ -156,6 +169,15 @@ matrix0.pyrocufflink.blue
 
 [victoria-metrics]
 
+[victoria-metrics:children]
+metricspi
+
+[vmagent:children]
+victoria-metrics
+
+[vmalert:children]
+metricspi
+
 [vm-hosts]
 
 [wheelhost]