function proxyOptions(config) {
  let proxyUrl
  if (config.proxy) {
    proxyUrl = config.proxy
  } else {
    proxyUrl = 'https://'
    let proxyAuth = config.proxy_user
    if (config.proxy_pass !== '') {
      proxyAuth += ':' + config.proxy_pass
      proxyUrl += `${proxyAuth}@`
    }

    proxyUrl += `${config.proxy_host || 'localhost'}:${config.proxy_port || 80}`
  }

  return proxyUrl
}
Returns an HTTP-agent provided by the https-proxy-agent
NPM package with configuration suitable for working via
the configured newrelic-agent's proxy configuration.

Include keep-alive configuration, but ultimately its up
to the proxy server as to how its connection is made
with New Relic's servers.

@param config