Marathon SD: Set port index label

The changes [1][] to Marathon service discovery to support multiple
ports mean that Prometheus now attempts to scrape all ports belonging to
a Marathon service.

You can use port definition or port mapping labels to filter out which
ports to scrape but that requires service owners to update their
Marathon configuration.

To allow for a smoother migration path, add a
`__meta_marathon_port_index` label, whose value is set to the port's
sequential index integer. For example, PORT0 has the value `0`, PORT1
has the value `1`, and so on.

This allows you to support scraping both the first available port (the
previous behaviour) in addition to ports with a `metrics` label.

For example, here's the relabel configuration we might use with
this patch:

    - action: keep
      source_labels: ['__meta_marathon_port_definition_label_metrics', '__meta_marathon_port_mapping_label_metrics', '__meta_marathon_port_index']
      # Keep if port mapping or definition has a 'metrics' label with any
      # non-empty value, or if no 'metrics' port label exists but this is the
      # service's first available port
      regex: ([^;]+;;[^;]+|;[^;]+;[^;]+|;;0)

This assumes that the Marathon API returns the ports in sorted order
(matching PORT0, PORT1, etc), which it appears that it does.

[1]: https://github.com/prometheus/prometheus/pull/2506

discovery/marathon/marathon.go

@@ -20,6 +20,7 @@ import (
 	"math/rand"
 	"net"
 	"net/http"
+	"strconv"
 	"strings"
 	"time"
 
@@ -43,6 +44,9 @@ const (
 	appLabel model.LabelName = metaLabelPrefix + "app"
 	// imageLabel is the label that is used for the docker image running the service.
 	imageLabel model.LabelName = metaLabelPrefix + "image"
+	// portIndexLabel is the integer port index when multiple ports are defined;
+	// e.g. PORT1 would have a value of '1'
+	portIndexLabel model.LabelName = metaLabelPrefix + "port_index"
 	// taskLabel contains the mesos task name of the app instance.
 	taskLabel model.LabelName = metaLabelPrefix + "task"
 
@@ -323,6 +327,7 @@ func targetsForApp(app *App) []model.LabelSet {
 			target := model.LabelSet{
 				model.AddressLabel: model.LabelValue(targetAddress),
 				taskLabel:          model.LabelValue(t.ID),
+				portIndexLabel:     model.LabelValue(strconv.Itoa(i)),
 			}
 			if i < len(app.PortDefinitions) {
 				for ln, lv := range app.PortDefinitions[i].Labels {