Gracefully handle shutdowns

[Pokom]

In the exporter.go, we launch our webserver and do not gracefully handle shutdowns:

cloudcost-exporter/cmd/exporter/exporter.go

Lines 90 to 93 in 04122bb

	if err = http.ListenAndServe(cfg.Server.Address, nil); err != nil {
	log.Printf("Error listening and serving: %s", err)
	os.Exit(1)
	}

The implications is that if k8s decides to shutdown the application mid scrape, the process will just shutdown before the scrape can finish. We should add a signal handler that listens for sigterm and sigexit signals and propagate the cancel through the contexts. If you're not familiar with this, check out this stackoverflow answer: https://stackoverflow.com/a/40041517