Fixed metric name.

Add metric description.
Add test.

dbms/src/Common/ProfileEvents.cpp

@@ -114,7 +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(CountOfMerges, "") \
+    M(Merge, "Number of launches 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

@@ -39,7 +39,7 @@ namespace ProfileEvents
     extern const Event MergedRows;
     extern const Event MergedUncompressedBytes;
     extern const Event MergesTimeMilliseconds;
-    extern const Event CountOfMerges;
+    extern const Event Merge;
 }
 
 namespace CurrentMetrics
@@ -511,7 +511,7 @@ public:
         if (stage.is_first)
         {
             ProfileEvents::increment(ProfileEvents::MergedRows, value.read_rows);
-            ProfileEvents::increment(ProfileEvents::CountOfMerges);
+            ProfileEvents::increment(ProfileEvents::Merge);
         }
         updateWatch();
 

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

+1

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

+DROP TABLE IF EXISTs new_table_test;
+DROP TABLE IF EXISTS check_table_test;
+
+CREATE TABLE IF NOT EXISTS new_table_test(name String) ENGINE = MergeTree Order By name;
+CREATE TABLE IF NOT EXISTS 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;