storage: add _num_rounds_window_compaction to probe

A metric that measures the number of rounds of sliding window compaction
that have been driven to completion.

src/v/storage/disk_log_impl.cc

@@ -522,7 +522,7 @@ segment_set disk_log_impl::find_sliding_range(
           config().ntp(),
           _last_compaction_window_start_offset.value(),
           _segs.front()->offsets().get_base_offset());
-
+        _probe->add_sliding_window_round_complete();
         _last_compaction_window_start_offset.reset();
     }
 
@@ -687,7 +687,7 @@ ss::future<bool> disk_log_impl::sliding_window_compact(
           "{}, resetting sliding window start offset",
           config().ntp(),
           idx_start_offset);
-
+        _probe->add_sliding_window_round_complete();
         next_window_start_offset.reset();
     }
 

src/v/storage/probe.cc

@@ -215,6 +215,12 @@ void probe::setup_metrics(const model::ntp& ntp) {
           sm::description("Number of segments that have been verified through "
                           "the compaction process to be tombstone free."),
           labels),
+        sm::make_counter(
+          "complete_sliding_window_rounds",
+          [this] { return _num_rounds_window_compaction; },
+          sm::description("Number of rounds of sliding window compaction that "
+                          "have been driven to completion."),
+          labels),
       },
       {},
       {sm::shard_label, partition_label});

src/v/storage/probe.h

@@ -102,6 +102,9 @@ class probe {
     void add_segment_marked_tombstone_free() {
         ++_segments_marked_tombstone_free;
     }
+    void add_sliding_window_round_complete() {
+        ++_num_rounds_window_compaction;
+    }
 
     void batch_parse_error() { ++_batch_parse_errors; }
 
@@ -146,6 +149,7 @@ class probe {
     uint64_t _tombstones_removed = 0;
     uint64_t _segment_cleanly_compacted = 0;
     uint64_t _segments_marked_tombstone_free = 0;
+    uint64_t _num_rounds_window_compaction = 0;
 
     ssize_t _compaction_removed_bytes = 0;