Add toolkit package

[SuperQ]

Implement a top level package to make it easy to bootstrap an exporter.

• Move the flag package to the top level.
• Add support for --web.telemetry-path flag. Defaults to/metrics.
• Add a self-check function to the web FlagConfig.

[SuperQ]

Here's what a basic startup looks like:

	landingConfig := web.LandingConfig{
		Name:        "Node Exporter",
		Description: "Prometheus Node Exporter",
		Version:     version.Info(),
	}
	toolkitConfig := toolkit.Config{
		MetricsHandler: newHandler(!*disableExporterMetrics, *maxRequests, logger),
		FlagConfig:     toolkitFlags,
		LandingConfig:  landingConfig,
		Logger:         logger,
	}
	if err := toolkit.Run(toolkitConfig); err != nil {
		level.Error(logger).Log("err", err)
		os.Exit(1)
	}

[SuperQ]

Still some work to do, tests, etc. But what do you think of this concept so far?

[SuperQ]

Ok, new example setup. There's an annoying chicken-and-egg problem with the MetricsHandler, since if we want to pass a logger to the handler, we need to New() first.

  toolkitConfig := toolkit.Config{
    Name:           "Node Exporter",
    Description:    "Prometheus Node Exporter",
    DefaultAddress: ":9100",
  }
  t := toolkit.New(toolkitConfig)
  t.SetMetricsHandler(newHandler(!*disableExporterMetrics, *maxRequests, t.Logger))

  if err := t.Run(); err != nil {
    level.Error(t.Logger).Log("err", err)
    os.Exit(1)
  }

[SuperQ]

One solution, I could implement part of the node_exporter's handler into this. One of the nice features is that it would implement the --web.disable-exporter-metrics flag by default, which would make a lot of users happy.