Add Changelog entry for cross-project label fix

CHANGELOG

@@ -4,6 +4,7 @@ v 8.8.0 (unreleased)
 
 v 8.7.0 (unreleased)
   - The number of InfluxDB points stored per UDP packet can now be configured
+  - Fix error when cross-project label reference used with non-existent project
   - Transactions for /internal/allowed now have an "action" tag set
   - Method instrumentation now uses Module#prepend instead of aliasing methods
   - Repository.clean_old_archives is now instrumented