Add metrics to the `PSR2.ControlStructures.SwitchDeclaration` sniff

[jrfnl]

Description

Recreation of upstream PR https://github.com/squizlabs/PHP_CodeSniffer/pull/1542:

I've explicitly not added metrics for:

• lowercase keyword as those are already recorded by the Generic.PHP.LowerCaseKeyword sniff
• break/return not on a line by itself as that is already recorded by the Generic.Formatting.DisallowMultipleStatements sniff

Resolves squizlabs/PHP_CodeSniffer#1541

Please let me know if there is anything you'd like me to change (phrasing and such).

Suggested changelog entry

PSR2/SwitchDeclaration: the sniff can now provide some metric on space usage in switch structures for a code base.