Update metadata instead of repace in _field helper

[daglofthus]

This will allow for a simple combination of protobuf messages and other metadata, like dataclass json custom encoders/decoders

[coveralls]

Pull Request Test Coverage Report for Build 4860070471

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 94.891%

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Totals
Change from base Build 4797148873:	0.08%
Covered Lines:	650
Relevant Lines:	685

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💛 - Coveralls

[eigenein]

Thank you!

[eigenein]

Published in 2.3.0