[exporter/datadog] Add Kubernetes hostname provider

[mx-psi]

Description:

• Add Kubernetes hostname provider for the preview hostname logic.
• Fixes some issues on the chain hostname provider (will add more tests on a future PR)

Link to tracking Issue: #10424

Testing: Added unit tests. TBD: end to end tests

[dashpole]

Why not use resource detectors, and then reference resource attributes in this exporter? It seems like this is reimplementing most resource detection code.

[mx-psi]

Why not use resource detectors, and then reference resource attributes in this exporter? It seems like this is reimplementing most resource detection code.

@dashpole That's a fair question. We have tried to rely as much as possible on the resource detection processor/k8sattributes processor so far (e.g. our current implementation requires you to use the k8sattributes processor if on Kubernetes for certain environments), but with this approach it's not possible to support some setups without passing significant complexity to users. Our exporter needs to send a payload with information about the 'host' in which it is running, but when information like k8s.node.name comes in a metrics/traces/logs payload, it's not possible to know if it comes from the host in which the Collector is running or from a remote host.

That said, I am trying to not duplicate code (see #10376) and will do future refactors to reduce code duplication more, so that in that sense the duplication is reduced as much as possible.