[exporter/datadog] Decouple Config structs from internal components

[mx-psi]

Description:

Decouple config.Config and related structs from internal components. In particular:

• Remove reference to Config on GetHost
• Remove reference to Config on ProcessMetrics
• Decouple metadata pusher configuration from main configuration

This is so that we can move config to the main package while avoiding cycles.

This is an internal refactor with no public changes.

Link to tracking Issue: #8373

Testing:

• amended unit tests.
• end to end testing of host metadata.
• check that the fallback hostname on the configuration is picked up.

[KSerrania]

The PusherConfig objects being abbreviated mcfg is slightly confusing, but otherwise LGTM.

[mx-psi]

The PusherConfig objects being abbreviated mcfg is slightly confusing, but otherwise LGTM.

I changed that, originally I named it MetadataPusherConfig, but the linters don't like that 😄

[mx-psi]

Marking as ready-to-merge since it has a codeowner approval