Field-Level Metadata Removed

[ers81239]

Describe the bug
Field level metadata is removed from the catalog when discovery is performed

To Reproduce
Steps to reproduce the behavior:

1. Provide field-level metadata in the catalog, for example:

{
          'breadcrumb': ['properties', 'char_name'],
          'metadata': {
	          'arbitrary_field_metadata': 'should be preserved'
          }
}

2. Run the tap

This affects at least Meltano users who use the meltano select feature to populate the selected metadatum to indicate whether fields should be copied or not. In that case, the selected field is being stripped and therefore ignored and all fields get copied regardless.

Expected behavior
Field-level metadata should be preserved through the stream_utils.refresh_streams_schema() with only discoverable metadata being updated/added.

Your environment

• Version of tap: master
• Version of python: 3.8

Additional context
I'm using the tap with meltano and setting a field metadatum: selected: True, however this is stripped from the metadata and in turn not honored by the tap.