Log why issue is not marked as stale (#301)

This might help to understand some situations as described in https://github.com/actions/stale/issues/299.

src/IssueProcessor.ts

@@ -283,6 +283,10 @@ export class IssueProcessor {
         );
         await this._markStale(issue, staleMessage, staleLabel, skipMessage);
         issue.isStale = true; // this issue is now considered stale
+      } else if (!issue.isStale) {
+        issueLogger.info(
+          `Not marking as stale: shouldBeStale=${shouldBeStale}, shouldMarkAsStale=${shouldMarkAsStale}`
+        );
       }
 
       // process the issue if it was marked stale