fix: retrieve constructors

[dandhlee]

Because of a Sphinx configuration set up on the Client libraries, Sphinx will try and combine the constructor and the Class definition together. However, from the Plugin side this is encountered as two separate docstrings with same name and _type == CLASS. Since we tried avoiding duplicate entries, this ended up getting rid of the constructors.

Retrieved constructors by identifying them (on their second encounter) as a method. The original class definition will retain much information about it as it can, with minimal overlap between the two. There can be an overlap for the constructor syntax showing both on the class header definition and the constructor method definition, I will ask for feedback on whether this should be okay or not.

As well, removing arguments with empty var_type or description. Missing either two likely results from a typo or from incomplete documentation which should be brought up with the respective client library owners.

Fixes b/208869740, b/208866895.

• Tests pass: tests will be part of the goldens for work to be done in Improve testing infrastructure to verify the output #44.

[busunkim96]

As well, removing arguments with empty var_type or description. Missing either two likely results from a typo or from incomplete documentation which should be brought up with the respective client library owners.

Could you share an example of a docstring where this would happen?

[dandhlee]

Missing var_type from an extra character typo, which results in the type not getting parsed properly.

Missing entries and therefore showing up as undefined in the doc: https://github.com/googleapis/python-aiplatform/blob/main/google/cloud/aiplatform/training_jobs.py#L3958-L3962. Trying to handle this more gracefully by not including this in the doc rather than showing them as None(.