This avoids the need to use a proxy, or some other means, to connect to
Prometheus. This is disabled by default and can be enabled by setting
enable_prometheus_server_external to true.

Change-Id: Ia0af044ff436c2a204b357750a16ff49fcdfec45

Change-Id: If42b7324b58851a4f0beb3a80c70ca2c9dd729af