[BUG] Custom metadata columns are always formatted using toString() in the message browser.

[rogin]

Describe the bug
I'm migrating popular requests from NextGen's repo. This was issue #6203

When using a non-STRING type for a custom Metadata, the dashboard should use a readable format instead of using the default "toString()", which sometime results in unreadable information.

Additional context
I or another can copy over the details is-simon provided. It may be undeleting a few lines to get this working again.

[sbraconnier]

I'll take care of this one...