Add merge metric

dbms/src/Common/ProfileEvents.cpp

@@ -114,6 +114,7 @@
     M(SelectedRanges, "Number of (non-adjacent) ranges in all data parts selected to read from a MergeTree table.") \
     M(SelectedMarks, "Number of marks (index granules) selected to read from a MergeTree table.") \
     \
+    M(Merge, "Number of launched background merges.") \
     M(MergedRows, "Rows read for background merges. This is the number of rows before merge.") \
     M(MergedUncompressedBytes, "Uncompressed bytes (for columns as they stored in memory) that was read for background merges. This is the number before merge.") \
     M(MergesTimeMilliseconds, "Total time spent for background merges.")\

dbms/src/Storages/MergeTree/MergeTreeDataMergerMutator.cpp

@@ -39,6 +39,7 @@ namespace ProfileEvents
     extern const Event MergedRows;
     extern const Event MergedUncompressedBytes;
     extern const Event MergesTimeMilliseconds;
+    extern const Event Merge;
 }
 
 namespace CurrentMetrics
@@ -508,7 +509,10 @@ class MergeProgressCallback
     {
         ProfileEvents::increment(ProfileEvents::MergedUncompressedBytes, value.read_bytes);
         if (stage.is_first)
+        {
             ProfileEvents::increment(ProfileEvents::MergedRows, value.read_rows);
+            ProfileEvents::increment(ProfileEvents::Merge);
+        }
         updateWatch();
 
         merge_entry->bytes_read_uncompressed += value.read_bytes;

dbms/tests/queries/0_stateless/01014_count_of_merges_metrics.reference

@@ -0,0 +1 @@
+1

dbms/tests/queries/0_stateless/01014_count_of_merges_metrics.sql

@@ -0,0 +1,14 @@
+DROP TABLE IF EXISTS new_table_test;
+DROP TABLE IF EXISTS check_table_test;
+
+CREATE TABLE new_table_test(name String) ENGINE = MergeTree ORDER BY name;
+INSERT INTO new_table_test VALUES ('test');
+CREATE TABLE check_table_test(value1 UInt64, value2 UInt64) ENGINE = MergeTree ORDER BY tuple();
+INSERT INTO check_table_test (value1) SELECT value FROM system.events WHERE event = 'Merge';
+OPTIMIZE TABLE new_table_test FINAL;
+INSERT INTO check_table_test (value2) SELECT value FROM system.events WHERE event = 'Merge';
+SELECT count() FROM check_table_test WHERE value2 > value1;
+
+
+DROP TABLE new_table_test;
+DROP TABLE check_table_test;