Match addresses with or without declared ports.

This change updates port relabeling for pod and service discovery so the
relabeling regex matches addresses with or without declared ports. As
well, this change uses a consistent style in the replacement pattern
for the two expressions.

Previously, for both services or pods that did not have declared ports, the
relabel config regex would fail to match:

    __meta_kubernetes_service_annotation_prometheus_io_port
    regex: (.+)(?::\d+);(\d+)

    __meta_kubernetes_pod_annotation_prometheus_io_port
    regex: (.+):(?:\d+);(\d+)

Both regexes expected a <host>:<port> pattern.

The new regex matches addresses with or without declared ports by making
the :<port> pattern optional.

    __meta_kubernetes_service_annotation_prometheus_io_port
    __meta_kubernetes_pod_annotation_prometheus_io_port
    regex: (.+)(?::\d+)?;(\d+)

documentation/examples/prometheus-kubernetes.yml

@@ -108,7 +108,7 @@ scrape_configs:
   - source_labels: [__address__, __meta_kubernetes_service_annotation_prometheus_io_port]
     action: replace
     target_label: __address__
-    regex: (.+)(?::\d+);(\d+)
+    regex: (.+)(?::\d+)?;(\d+)
     replacement: $1:$2
   - action: labelmap
     regex: __meta_kubernetes_service_label_(.+)
@@ -174,8 +174,8 @@ scrape_configs:
     regex: (.+)
   - source_labels: [__address__, __meta_kubernetes_pod_annotation_prometheus_io_port]
     action: replace
-    regex: (.+):(?:\d+);(\d+)
-    replacement: ${1}:${2}
+    regex: (.+)(?::\d+)?;(\d+)
+    replacement: $1:$2
     target_label: __address__
   - action: labelmap
     regex: __meta_kubernetes_pod_label_(.+)