[docker] fix net metrics in host mode

[hkaj]

Note: Please remember to review the Datadog Contribution Guidelines
if you have not yet done so.

What does this PR do?

If a container is in host mode we shouldn't call socket.inet_aton on its network gateway because it's empty and will fail, resulting in a warning in the integration. We can't not report the metrics either because existing behavior reports network metrics even in host mode (so network metrics are actually for the whole host). So let's keep the same behavior but avoid the buggy call to inet_aton.

Motivation

Getting rid of a warning.

Testing Guidelines

Run the agent in host mode (or any other container on the same machine), see that the warning disappeared compared to the RC, replaced with a debug log.

[truthbk]

Looks good. My only question, or maybe consideration, is: could we have other interfaces in the mapping that should maybe be accounted for in host mode other than eth0?

[truthbk]

We can discuss tomorrow, this is good regardless as far as I'm concerned.