feat: Introduce switch for envelope v1 metrics

[b1tamara]

• Read the Contributing document.

Summary

• Implement a switch to enable / disable metrics reported using Envelope V1
• Introduce a new gorouter prometheus property "enabled" which has been automatically set to true if prometheus.port is set.
• Do not exclude metrics reporting by prometheus and using Envelope V1.

Backward Compatibility

Breaking Change? No

[ctlong]

If you want the prometheus metrics to be scraped by the Prom Scraper agent, then you'll need to configure a prom_scraper_config.yml. You can read more about that here.