Check switches on syntax.Expr for exhaustiveness.

[jeschkies]

What this PR does / why we need it:
In preparation to serlialize the LogQL AST as JSON we are declaring the syntax.Expr as a "sum type" and test all switch statements on it to be exhaustive.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
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• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
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