Fix write stall stats dump format (#11445)

Summary:
I noticed in https://github.com/facebook/rocksdb/issues/11426 there is a missing line break. Before this PR the output looked like

```
$ ./db_bench -stats_per_interval=1 -stats_interval=100000
...
Write Stall (count): cf-l0-file-count-limit-delays-with-ongoing-compaction: 0, cf-l0-file-count-limit-stops-with-ongoing-compaction: 0, l0-file-count-limit-delays: 0, l0-file-count-limit-stops: 0, memtable-limit-delays: 0, memtable-limit-stops: 0, pending-compaction-bytes-delays: 0, pending-compaction-bytes-stops: 0, total-delays: 0, total-stops: 0, Block cache LRUCache@0x7f8695831b50#2766536 capacity: 32.00 MB seed: 1155354975 usage: 0.09 KB table_size: 1024 occupancy: 1 collections: 1 last_copies: 0 last_secs: 9.3e-05 secs_since: 2
...
Write Stall (count): write-buffer-manager-limit-stops: 0, num-running-compactions: 0
...
```

After this PR it looks like

```
$ ./db_bench -stats_per_interval=1 -stats_interval=100000
...
Write Stall (count): cf-l0-file-count-limit-delays-with-ongoing-compaction: 0, cf-l0-file-count-limit-stops-with-ongoing-compaction: 0, l0-file-count-limit-delays: 0, l0-file-count-limit-stops: 0, memtable-limit-delays: 0, memtable-limit-stops: 0, pending-compaction-bytes-delays: 0, pending-compaction-bytes-stops: 0, total-delays: 0, total-stops: 0
Block cache LRUCache@0x7f8e0d231b50#2736585 capacity: 32.00 MB seed: 920433955 usage: 0.09 KB table_size: 1024 occupancy: 1 collections: 1 last_copies: 1 last_secs: 6.5e-05 secs_since: 4
...
Write Stall (count): write-buffer-manager-limit-stops: 0
num-running-compactions: 0
...
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11445

Reviewed By: hx235

Differential Revision: D45844752

Pulled By: ajkr

fbshipit-source-id: 1c708cb05b6e270922ac2fa95f5d011f273347eb

db/internal_stats.cc

@@ -1688,8 +1688,16 @@ void InternalStats::DumpDBStatsWriteStall(std::string* value) {
   std::ostringstream str;
   str << "Write Stall (count): ";
 
-  for (const auto& name_and_stat : write_stall_stats_map) {
-    str << name_and_stat.first << ": " << name_and_stat.second << ", ";
+  for (auto write_stall_stats_map_iter = write_stall_stats_map.begin();
+       write_stall_stats_map_iter != write_stall_stats_map.end();
+       write_stall_stats_map_iter++) {
+    const auto& name_and_stat = *write_stall_stats_map_iter;
+    str << name_and_stat.first << ": " << name_and_stat.second;
+    if (std::next(write_stall_stats_map_iter) == write_stall_stats_map.end()) {
+      str << "\n";
+    } else {
+      str << ", ";
+    }
   }
   *value = str.str();
 }
@@ -1871,8 +1879,16 @@ void InternalStats::DumpCFStatsWriteStall(std::string* value,
   std::ostringstream str;
   str << "Write Stall (count): ";
 
-  for (const auto& name_and_stat : write_stall_stats_map) {
-    str << name_and_stat.first << ": " << name_and_stat.second << ", ";
+  for (auto write_stall_stats_map_iter = write_stall_stats_map.begin();
+       write_stall_stats_map_iter != write_stall_stats_map.end();
+       write_stall_stats_map_iter++) {
+    const auto& name_and_stat = *write_stall_stats_map_iter;
+    str << name_and_stat.first << ": " << name_and_stat.second;
+    if (std::next(write_stall_stats_map_iter) == write_stall_stats_map.end()) {
+      str << "\n";
+    } else {
+      str << ", ";
+    }
   }
 
   if (total_stall_count) {