Fix NewRelicContextFormatter bug

[hmstepanek]

Overview

In v9.5.0, a bug in the NewRelicContextFormatter was introduced where "message" was added to the default set of keys to exclude. This made the default key list (default keys + 1) in length. Since the logic check was based on the length, rather than presence of certain keys, anything that had one extra key than the default would not have that extra key added to the ouput because the length would match the (default keys + 1) length.

The length check that was previously used was a bit of an odd choice. Replace this with a check for keys not in the default keys list.

[github-actions[bot]]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	bandit	1		0	4.63s
✅ PYTHON	black	2	0	0	0.86s
✅ PYTHON	flake8	2		0	0.46s
✅ PYTHON	isort	2	0	0	0.25s
✅ PYTHON	pylint	2		0	3.69s

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