Support new lines in struct tags

[bwplotka]

Requires custom structTag.Get implementation as mentioned here: https://stackoverflow.com/questions/51984366/how-to-split-long-struct-tags-in-golang

The Go totally works with newlines in tags:

	var err error
	cfg := struct {
		ConfigFile  string `flagarize:"name=config.file|help=Prometheus configuration file path.|default=prometheus.yml"`
		ExternalURL string `flagarize:"name=web.external-url|help=The URL under which Prometheus is externally reachable (for example, if Prometheus is served via a reverse proxy). \
Used for generating relative and absolute links back to Prometheus itself. If the URL has a path portion, it will be used to prefix all HTTP endpoints served by Prometheus. If omitted, relevant URL components will be derived automatically.|placeholder=<URL>"`
		StoragePath          string         `flagarize:"name=storage.tsdb.path|help=Base path for metrics storage.|default=data/"`
		RemoteFlushDeadline  model.Duration `flagarize:"name=storage.remote.flush-deadline|help=How long to wait flushing sample on shutdown or config reload.|default=1m|placeholder=<duration>"`
		RulesOutageTolerance model.Duration `flagarize:"name=rules.alert.for-outage-tolerance|help=Max time to tolerate prometheus outage for restoring \"for\" state of alert.|default=1h"`
		RulesForGracePeriod  model.Duration `flagarize:"name=rules.alert.for-grace-period|help=Minimum duration between alert and restored \"for\" state. This is maintained only for alerts with configured \"for\" time greater than grace period.|default=10m"`
		RulesResendDelay     model.Duration `flagarize:"name=rules.alert.resend-delay|help=Minimum amount of time to wait before resending an alert to Alertmanager.|default=1m"`
		LookbackDelta        model.Duration `flagarize:"name=query.lookback-delta|help=The maximum lookback duration for retrieving metrics during expression evaluations and federation.|default=5m"`
		QueryTimeout         model.Duration `flagarize:"name=query.timeout|help=Maximum time a query may take before being aborted.|default=2m"`
		QueryConcurrency     int            `flagarize:"name=query.max-concurrency|help=Maximum number of queries executed concurrently.|default=20"`
		QueryMaxSamples      int            `flagarize:"name=query.max-samples|help=Maximum number of samples a single query can load into memory. Note that queries will fail if \
they try to load more samples than this into memory, so this also limits the number of samples a query can return.|default=50000000"`

		Web      web.Options
		Notifier notifier.Options
		TSDB     tsdbOptions
		PromLog  promlog.Config
	}{
		Notifier: notifier.Options{
			Registerer: prometheus.DefaultRegisterer,
		},
		Web: web.Options{
			Registerer: prometheus.DefaultRegisterer,
			Gatherer:   prometheus.DefaultGatherer,

			CORSOriginFlagarizeHelp: `Regex for CORS origin. It is fully anchored. Example: 'https?://(domain1|domain2)\.com'`,
		},
		TSDB: tsdbOptions{
			RetentionDurationFlagarizeHelp: "How long to retain samples in storage. When this flag is set it overrides \"storage.tsdb.retention\". If neither this flag nor \"storage.tsdb.retention\" nor \"storage.tsdb.retention.size\" is set, the retention time defaults to " + defaultRetentionString + ". Units Supported: y, w, d, h, m, s, ms.",
		},
		PromLog: promlog.Config{},
	}

	a := kingpin.New(filepath.Base(os.Args[0]), "The Prometheus monitoring server")
	a.Version(version.Print("prometheus"))
	a.HelpFlag.Short('h')

	if err := flagarize.Flagarize(a, &cfg); err != nil {
		fmt.Fprintln(os.Stderr, err)
		os.Exit(2)
	}

	if _, err := a.Parse(os.Args[1:]); err != nil {
		fmt.Fprintln(os.Stderr, errors.Wrapf(err, "Error parsing commandline arguments"))
		a.Usage(os.Args[1:])
		os.Exit(2)
	}