storage: Omit empty fields from rangelog json

This shrinks the size of the info field in rangelog entries.

gogoproto automatically adds the omitempty json tag for all proto3
fields (that aren't explicitly marked as nullable with a gogoproto tag),
but it can't do so if the user specifies a custom jsontag. I don't know
for sure why we initially added these, but now I can't remove them
without messing up backwards compatibility with the old json field
names, so just add the necessary omitempty annotations.

Helps with range log size as related to cockroachdb#21260.

Release note (sql change): Reduced size of entries stored in the
system.rangelog table by not storing empty JSON fields.

pkg/storage/log.proto

@@ -33,15 +33,15 @@ enum RangeLogEventType {
 
 message RangeLogEvent {
   message Info {
-      roachpb.RangeDescriptor updated_desc = 1 [(gogoproto.jsontag) = "UpdatedDesc"];
-      roachpb.RangeDescriptor new_desc = 2 [(gogoproto.jsontag) = "NewDesc"];
-      roachpb.ReplicaDescriptor added_replica = 3 [(gogoproto.jsontag) = "AddReplica"];
-      roachpb.ReplicaDescriptor removed_replica = 4 [(gogoproto.jsontag) = "RemovedReplica"];
+      roachpb.RangeDescriptor updated_desc = 1 [(gogoproto.jsontag) = "UpdatedDesc,omitempty"];
+      roachpb.RangeDescriptor new_desc = 2 [(gogoproto.jsontag) = "NewDesc,omitempty"];
+      roachpb.ReplicaDescriptor added_replica = 3 [(gogoproto.jsontag) = "AddReplica,omitempty"];
+      roachpb.ReplicaDescriptor removed_replica = 4 [(gogoproto.jsontag) = "RemovedReplica,omitempty"];
       string reason = 5 [
-        (gogoproto.jsontag) = "Reason",
+        (gogoproto.jsontag) = "Reason,omitempty",
         (gogoproto.casttype) = "RangeLogEventReason"
       ];
-      string details = 6 [(gogoproto.jsontag) = "Details"];
+      string details = 6 [(gogoproto.jsontag) = "Details,omitempty"];
   }
 
   google.protobuf.Timestamp timestamp = 1 [