Add a helper to generate legacy configuration

This moves the DefaultConfig method out into a helper to generate legacy
configuration.  Passing in a config file is now required.

cmd/config-gen/main.go

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"github.com/spf13/cobra"
+	yaml "gopkg.in/yaml.v2"
+
+	"github.com/directxman12/k8s-prometheus-adapter/cmd/config-gen/utils"
+)
+
+func main() {
+	var labelPrefix string
+	var rateInterval time.Duration
+
+	cmd := &cobra.Command{
+		Short: "Generate a config matching the legacy discovery rules",
+		Long: `Generate a config that produces the same functionality
+as the legacy discovery rules. This includes discovering metrics and associating
+resources according to the Kubernetes instrumention conventions and the cAdvisor
+conventions, and auto-converting cumulative metrics into rate metrics.`,
+		RunE: func(c *cobra.Command, args []string) error {
+			cfg := utils.DefaultConfig(rateInterval, labelPrefix)
+			enc := yaml.NewEncoder(os.Stdout)
+			if err := enc.Encode(cfg); err != nil {
+				return err
+			}
+			return enc.Close()
+		},
+	}
+
+	cmd.Flags().StringVar(&labelPrefix, "label-prefix", "",
+		"Prefix to expect on labels referring to pod resources.  For example, if the prefix is "+
+			"'kube_', any series with the 'kube_pod' label would be considered a pod metric")
+	cmd.Flags().DurationVar(&rateInterval, "rate-interval", 5*time.Minute,
+		"Period of time used to calculate rate metrics from cumulative metrics")
+
+	if err := cmd.Execute(); err != nil {
+		fmt.Fprintf(os.Stderr, "Unable to generate config: %v\n", err)
+		os.Exit(1)
+	}
+}