`Field.try_merge` should have optional metadata merge

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
Merging fields is useful but the implicit merge of metadata is something that should be opt-in. At the moment we have to undo this to get the required effect.

Start by locating Field::try_merge in the arrow-rs source and inspect how metadata is currently merged. Determine the existing tests for field merging, then define coverage showing that metadata merging can be enabled or omitted as requested; done means callers can opt out of the implicit metadata merge.

Written by the indexing model from the issue text.