fix: support configuration of HTTP server address (#1365)
fix: support configuration of HTTP server address (#1365)

In Kubernetes, the convention is to bind HTTP probes and Prometheus endpoints to
0.0.0.0 (both lo and eth0). Since people might want to run this code on a GCE
VM, default to localhost, but otherwise support binding to both interfaces.

Fixes #1359.
GoogleCloudPlatform/cloud-sql-proxy,GoogleCloudPlatform/cloud-sql-proxy