diff --git a/src/mgr/DaemonHealthMetricCollector.cc b/src/mgr/DaemonHealthMetricCollector.cc
index 4b51100ee2e..53c0b78a690 100644
--- a/src/mgr/DaemonHealthMetricCollector.cc
+++ b/src/mgr/DaemonHealthMetricCollector.cc
@@ -30,6 +30,8 @@ class SlowOps final : public DaemonHealthMetricCollector {
     if (daemons.empty()) {
       return;
     }
+    // Note this message format is used in mgr/prometheus, so any change in format
+    // requires a corresponding change in the mgr/prometheus module.
     ostringstream ss;
     if (daemons.size() > 1) {
       if (daemons.size() > 10) {