The tests for Kubernetes SD rely on comparing target groups by first
serializing them to JSON. However, the target group MarshalJSON function
only serializes the __address__ label, which makes eliminates all other
labels from the comparison.
This commit implements a separate marshaling function intended for use in
Kubernetes SD tests. The function serializes all target labels, making
comparisons much more reliable. The commit also fixes all tests that
started to fail due to the newly introduced change.
Signed-off-by: Filip Petkovski <filip.petkovsky@gmail.com>