*: Add dbSizeInUse to StatusResposne

Existing dbSize shows physically allocated DB size and the backend
(boltdb) won't shrink it after a compaction until a user runs the defrag command.
The new dbSizeInUse shows the DB size that excludes free pages created
by compactions so that users can see the actual DB usage. dbSize >=
dbSizeInUse is always true.
Note that dbSizeInUse shows a page-based size and not byte level usage.

Documentation/dev-guide/apispec/swagger/rpc.swagger.json

@@ -2164,7 +2164,12 @@
       "type": "object",
       "properties": {
         "dbSize": {
-          "description": "dbSize is the size of the backend database, in bytes, of the responding member.",
+          "description": "dbSize is the size of the backend database physically allocated, in bytes, of the responding member.",
+          "type": "string",
+          "format": "int64"
+        },
+        "dbSizeInUse": {
+          "description": "dbSize is the size of the backend database logically in use, in bytes, of the responding member.",
           "type": "string",
           "format": "int64"
         },

etcdserver/api/v3rpc/maintenance.go

@@ -164,6 +164,7 @@ func (ms *maintenanceServer) Status(ctx context.Context, ar *pb.StatusRequest) (
 		RaftIndex:        ms.rg.Index(),
 		RaftTerm:         ms.rg.Term(),
 		RaftAppliedIndex: ms.rg.AppliedIndex(),
+		DbSizeInUse:      ms.bg.Backend().SizeInUse(),
 	}
 	if uint64(ms.rg.Leader()) == raft.None {
 		resp.Errors = append(resp.Errors, etcdserver.ErrNoLeader.Error())