[connector/count] Fix regression in basic config behavior

[kuiperda]

Description

See issue for details.

TLDR: count relies on Unmarshal to set default subconfigs, but this update changed confmap behavior to NOT call unmarshal for nil subconfigs like this one which count expects to work:

connector:
  count:

so no metrics are emitted, even though this is a basic example of usage from the readme.

A workaround is to set this instead:

connector:
  count: {}

but I think it's better to match the previous behavior by setting the default configs with createDefaultConfig and just overriding them in Unmarshal if custom metrics are defined.

Link to tracking issue

Fixes 41769

Testing

• Added unit tests. Tests relevant to this PR's changes fail if run on main. (Specifically tests 1-2)
• Manually tested the same configs covered by unit tests.
• There is a detailed test example in the issue.

Documentation

N/A, this fix makes the behavior match documentation again.