Add support for sending host tags for a period of time after start up.

[tobz]

Context

The Datadog Agent has the ability to enrich metrics with the host tags collected by the Agent for a configurable period of time after startup, after which the host tags are no longer automatically added and only the original tags, and non-host-specific enriched tags, are utilized.

This allows compensating for the backend delay where new hosts (from the perspective of the Datadog backend) don't yet have their tags fully resolved, and so the Datadog backend won't be automatically adding those tags to metrics being sent by the Agent. At some point (ideally) between when the new host comes online and the "send host tags automatically" logic turns itself off in the Agent, the backend kicks into action and starts doing this.

We should be able to emulate this in ADP as well, as it helps avoid gaps in the queryability of metrics when host tags are being used.