util/metric: add some commentary to the Metadata proto

Release note: None

Author: yuzefovich

pkg/util/metric/metric.proto

@@ -89,15 +89,21 @@ message Metadata {
     TTL = 15;
     EXPIRATIONS = 16;
   }
-  
+
   // if essential is true, the metric is required to be included in
   // a DB Console dashboard, in our public docs, and in all tsdump
   // exports.
+  //
+  // The initial list of essential metrics was based on the "100 essential
+  // metrics" that can be found at
+  // https://github.com/cockroachlabs/cockroachdb-runbook-template/blob/main/monitoring-alerts/monitoring-dashboard-custom.md
   required bool essential = 9 [(gogoproto.nullable) = false];
   // category is the dashboard category of this metric. This is
   // required if `essential` is true.
   required Category category = 11 [(gogoproto.nullable) = false];
   // how_to_use is an extended description of how to use this metric
   // with a running cluster. This is required if `essential` is true.
   required string how_to_use = 12 [(gogoproto.nullable) = false];
+
+  // Next ID: 10 (then 13).
 }