How to specify proxy url for hearbeat cloud.id

Hi,

How can proxy url be set so heartbeat can resolve cloud.id ?

Heartbeat is currently sitting behind corporate proxy, and unable to connect to elasticsearch cloud.

2020-10-11T02:14:06.182+0800	INFO	pipeline/output.go:93	Attempting to reconnect to backoff(elasticsearch(https://xxx.elastic-cloud.com:443)) with 5 reconnect attempt(s)
2020-10-11T02:14:06.182+0800	INFO	[publisher]	pipeline/retry.go:166	retryer: send wait signal to consumer
2020-10-11T02:14:06.182+0800	INFO	[publisher]	pipeline/retry.go:168	  done
2020-10-11T02:14:18.187+0800	WARN	transport/tcp.go:53	DNS lookup failure "xxx.elastic-cloud.com": lookup xxx.elastic-cloud.com: no such host

Setting environment variable HTTPS_PROXY works for me!