buildkitd: Fix HPA label selectors by splitting envoy cluster_name
Kubernetes did not like the label selector on cluster_name that
contained '|' delimited information.
Configure the "envoy-stats-monitor" PodMonitor to relabel scraped
metrics and split the cluster_name label into distinct host/port/name
labels.
Reconfigure HPA to select on cluster_name: "inbound" and
cluster_port: "1234" to only sum active connections to buildkitd.
Bug: T327416