Annotate and clarify that metadata keys should be strings (backport #11736)

[mergify]

This is an automatic backport of pull request #11736 done by Mergify.

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[coveralls]

Pull Request Test Coverage Report for Build 7818695601

• 0 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 19 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.004%) to 89.184%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	93.81%
crates/qasm2/src/lex.rs	6	91.18%
crates/qasm2/src/parse.rs	12	97.15%

Totals
Change from base Build 7818124522:	-0.004%
Covered Lines:	58806
Relevant Lines:	65938

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💛 - Coveralls