Clean up sample app

[JasonXZLiu]

Clean up sample app by:

• removing the expected_metrics endpoint (only used for integration tests)
• changing environment variables to command line flags
• use logging statements (instead of printing to stdout)

Logs:

$ go build .
$ ./prometheus-sample-app -listen_address=0.0.0.0:4567 -metric_count=100
2020/12/07 11:16:28 Serving on address: 0.0.0.0:4567
2020/12/07 11:16:28 Producing 100 metrics per type