Set INSTANCE name via environment variable

[zerosoul13]

Hello,

Today I was looking to use ${INSTANCE} environment variable but only found support for ${HOST}. The reason being that ${INSTANCE} environment variable is declared in the examples/k8s/deployment.yaml file.

    spec:
      containers:
      - env:
        - name: INSTANCE
          valueFrom:
            fieldRef:
              fieldPath: metadata.name

After reading the carbon-relay-ng code https://github.com/grafana/carbon-relay-ng/blob/master/cmd/carbon-relay-ng/carbon-relay-ng.go#L74, only found support for ${HOST}

func expandVars(in string) (out string) {
	switch in {
	case "HOST":
		hostname, _ := os.Hostname()
		// in case hostname is an fqdn or has dots, only take first part
		parts := strings.SplitN(hostname, ".", 2)
		return parts[0]
	case "GRAFANA_NET_ADDR":
		return os.Getenv("GRAFANA_NET_ADDR")
	case "GRAFANA_NET_API_KEY":
		return os.Getenv("GRAFANA_NET_API_KEY")
	case "GRAFANA_NET_USER_ID":
		return os.Getenv("GRAFANA_NET_USER_ID")
	default:
		return "$" + in
	}

Would you be open to supporting a new variable instead of using the default ${HOST}?

func expandVars(in string) (out string) {
	switch in {
        case "INSTANCE":
                return os.Getenv("INSTANCE") 
	case "HOST":
		hostname, _ := os.Hostname()
		// in case hostname is an fqdn or has dots, only take first part
		parts := strings.SplitN(hostname, ".", 2)
		return parts[0]
	case "GRAFANA_NET_ADDR":
		return os.Getenv("GRAFANA_NET_ADDR")
	case "GRAFANA_NET_API_KEY":
		return os.Getenv("GRAFANA_NET_API_KEY")
	case "GRAFANA_NET_USER_ID":
		return os.Getenv("GRAFANA_NET_USER_ID")
	default:
		return "$" + in
	}