plugin/health: clarify server label
Health overloaded metrics does not carry the server label. Explain why.
@@ -60,6 +60,9 @@ If monitoring is enabled (via the *prometheus* directive) then the following met
be a local operation it should be fast. A (large) increases in this duration indicates the
CoreDNS process is having trouble keeping up with its query load.
+Note that this metric *does not* have a `server` label, because being overloaded is a symptom of
+the running process, *not* a specific server.
+
## Examples
Run another health endpoint on http://localhost:8091.
