metrics-http-json-deep.rb: fix key normalization

[ushi-as]

String#tr accepts two strings as arguments. Not a RegExp.

See: https://ruby-doc.org/core-2.3.3/String.html#method-i-tr

It raises: no implicit conversion of Regexp into String at the moment.

Pull Request Checklist

General

• [ ] Update Changelog following the conventions laid out on Keep A Changelog

• [ ] Update README with any necessary configuration snippets

• [ ] Binstubs are created if needed

• [ ] RuboCop passes

• [ ] Existing tests pass

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• [ ] Tests

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Purpose

Known Compatablity Issues

[eheydrick]

Thanks for the PR. I ended up merging #65 which does virtually the same thing. The fix is in 2.0.2 which is now live on Rubygems.