https://github.com/prometheus/prometheus/blob/master/documentation/examples/prometheus-kubernetes.yml
# Example relabel to scrape only single, desired port for the pod
# based on pod "example.io/scrape_port = <port>" annotation.
# Note that __address__ is modified here, so if pod containers' ports
# are declared, they all will be ignored.
# - source_labels: [__address__, __meta_kubernetes_pod_annotation_example_io_scrape_port]
# action: replace
# regex: ([^:]+)(?::\d+)?;(\d+)
# replacement: $1:$2
# target_label: __address__
