rdbms contrib: pass connection to _add_metadata_columns

[Styren]

Description

The method _MetadataColumnsMixin#_add_metadata_columns accepts a connection parameter for creating the metadata columns for the CopyToTable task. However, the general CopyToTable instead sends a cursor while the redshift version sends the correct connection:

luigi/luigi/contrib/redshift.py

Lines 479 to 480 in cf2abbd

	if self.enable_metadata_columns:
	self._add_metadata_columns(connection)

Motivation and Context

Without this change you run into a AttributeError when you select enable_metadata_columns = True, when for example using the postgres CopyToTable.

  File "/usr/local/lib/python3.8/site-packages/luigi/contrib/rdbms.py", line 252, in init_copy
    self._add_metadata_columns(connection.cursor())
  File "/usr/local/lib/python3.8/site-packages/luigi/contrib/rdbms.py", line 103, in _add_metadata_columns
    cursor = connection.cursor()
AttributeError: 'psycopg2.extensions.cursor' object has no attribute 'cursor'

Have you tested this? If so, how?

Ran the contrib tests.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If closed, you may revisit when your time allows and reopen! Thank you for your contributions.

[dlstadther]

Apologies for massive delay. Stalebot comments on a bunch of Luigi PRs and that reminded me that I approved some PRs and never merged them. Going back through them now, updating them with master, and confirming that all their checks pass.