Fix misleading `:example_group` metadata deprecation warning.

Fixes #1562.
We need to silence it from the metadata filtering logic so that users don't get confusing deprecation warnings from a `describe AnyExampleGroup` call site.  (That happens because every time an example group is defined, it looks at configured metadata filterings and tries to apply the `include` configs, which, if that uses `:example_group`, will trigger this warning).  The thread local was the best way I could find to toggle this.