Generic/HereNowdocIdentifierSpacing: add metrics

This commit adds metrics to the new `Generic.WhiteSpace.HereNowdocIdentifierSpacing` sniff to allow for collecting statistical information on how often spacing is found between the `<<<` and the doc ID.

src/Standards/Generic/Sniffs/WhiteSpace/HereNowdocIdentifierSpacingSniff.php

@@ -48,9 +48,12 @@ public function process(File $phpcsFile, $stackPtr)
             && strpos($tokens[$stackPtr]['content'], "\t") === false
         ) {
             // Nothing to do.
+            $phpcsFile->recordMetric($stackPtr, 'Heredoc/nowdoc identifier', 'no space between <<< and ID');
             return;
         }
 
+        $phpcsFile->recordMetric($stackPtr, 'Heredoc/nowdoc identifier', 'space between <<< and ID');
+
         $error = 'There should be no space between the <<< and the heredoc/nowdoc identifier string';
         $data  = [$tokens[$stackPtr]['content']];