c/topic_table_probe: use btree_map in topic table probe

Replaced `absl::flat_hash_map` that was used in `topic_table_probe` with
the `btree_map` to avoid large allocations when we deal with large
number of topics.

Signed-off-by: Michal Maslanka <michal@redpanda.com>
(cherry picked from commit 471f036)

src/v/cluster/topic_table_probe.h

@@ -13,6 +13,7 @@
 #include "cluster/fwd.h"
 #include "ssx/metrics.h"
 
+#include <absl/container/btree_map.h>
 #include <absl/container/flat_hash_set.h>
 
 namespace cluster {
@@ -49,7 +50,7 @@ class topic_table_probe {
 
     const topic_table& _topic_table;
     model::node_id _node_id;
-    absl::flat_hash_map<model::topic_namespace, ss::metrics::metric_groups>
+    absl::btree_map<model::topic_namespace, ss::metrics::metric_groups>
       _topics_metrics;
     ss::metrics::metric_groups _internal_metrics;
     ss::metrics::metric_groups _public_metrics{