feature: allow to scrape metrics from node-agent

[lernett]

currently, node-agent overwrites listen addr to localhost if configured with MetricsEndpoint, which prevents scraping metrics from it https://github.com/coroot/coroot-node-agent/blob/c0c56bc4abf4ada73bae96a1df98befe77ea64b1/flags/flags.go#L93 not sure if it's done intentionally or not, but it in our use case it would be great to be able to collect metrics from node-agent directly along with its built-in remote write option