bounded_stats_queue: add missing const qualifiers

Most of the methods of this utility are effectively const.
Message-Id: <ed376ab74b6323cf770cc0a1314edbae0b16111e.1612953601.git.sarna@scylladb.com>

utils/bounded_stats_deque.hh

@@ -55,7 +55,7 @@ public:
         : _max_size(size) {
     }
 
-    int size() {
+    int size() const {
         return _deque.size();
     }
 
@@ -69,11 +69,11 @@ public:
         _sum += i;
     }
 
-    long sum() {
+    long sum() const {
         return _sum;
     }
 
-    double mean() {
+    double mean() const {
         return size() > 0 ? ((double) sum()) / size() : 0;
     }