[apollo-engine-reporting] Support proxy servers

[eberhara]

Hey guys,
Is there any way to set apollo engine reporting metrics to go through a proxy server?

Our environment in production does not have direct access to the internet, so we cannot proceed with apollo-engine metrics for now.

I saw in an older version that proxyUrl was supported, but I do not see it in the current apollo versions.

[eberhara]

Created a PR for that #1837

[eberhara]

Not sure why @n1ru4l removed the blocking label since this is indeed blocking our team from going to production. Anyway, I created a PR fixing that, and it would be cool if someone could take a look at it.