[receiver/dockerstats] Fix mistake where config has wrong key (V2 featuregate only)

[jamesmoessis]

trafficstars

Description:

The key in the config was "stats" but it should be "metrics" so it's consistent with documentation.md and other similar scrapers.

Link to tracking Issue: https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/9794

Testing:

I've updated the test so it validates the metrics key (this probably should've already been there).

[jamesmoessis]

FYI @MovieStoreGuy @rmfitzpatrick