Generic/UnnecessaryHeredoc: add metrics

This commit adds metrics to the new `Generic.Strings.UnnecessaryHeredoc` sniff to allow for collecting statistical information on how often a heredoc is used without expressions or interpolation.

src/Standards/Generic/Sniffs/Strings/UnnecessaryHeredocSniff.php

@@ -65,6 +65,7 @@ public function process(File $phpcsFile, $stackPtr)
                 || $bodyToken[0] === T_VARIABLE
             ) {
                 // Contains interpolation or expression.
+                $phpcsFile->recordMetric($stackPtr, 'Heredoc contains expression', 'yes');
                 return;
             }
 
@@ -73,10 +74,13 @@ public function process(File $phpcsFile, $stackPtr)
                 && $tokenizedBody[($ptr + 1)] === '$'
             ) {
                 // Contains interpolation or expression.
+                $phpcsFile->recordMetric($stackPtr, 'Heredoc contains expression', 'yes');
                 return;
             }
         }
 
+        $phpcsFile->recordMetric($stackPtr, 'Heredoc contains expression', 'no');
+
         $warning = 'Detected heredoc without interpolation or expressions. Use nowdoc syntax instead';
 
         $fix = $phpcsFile->addFixableWarning($warning, $stackPtr, 'Found');