Add support to Datadog Metrics destination for shipping to multiple endpoints.

[tobz]

Currently, the Datadog Metrics destination does not support multiple endpoints, only the "main" endpoint as configured by either site or dd_url. The Datadog Agent does support multiple endpoints through its general additional_endpoints configuration scheme, used across logs and metrics for configuring additional endpoints to dual (or triple! etc) ship to.

We should:

• add support for dual shipping (this should also use tower, but there's no existing tower middleware for fanning out a request, so we'll have to write that ourselves, and take care around making request cloning cheap)
• match the existing additional_endpoints configuration in the Datadog Agent's forwarder

[tobz]

As a note, since it just occurred to me: we'll want to make sure that this takes advantage of ReplayBody<B> which we utilize to efficiently support retries.