Add metric for number of tables being compacted (dgraph-io#1554)

Add an expVar to expose the metric for number of tables being compacted at
any point of time.

levels.go

@@ -488,6 +488,10 @@ func (s *levelsController) compactBuildTables(
 	topTables := cd.top
 	botTables := cd.bot
 
+	numTables := int64(len(topTables) + len(botTables))
+	y.NumCompactionTables.Add(numTables)
+	defer y.NumCompactionTables.Add(-numTables)
+
 	// Check overlap of the top level with the levels which are not being
 	// compacted in this compaction.
 	hasOverlap := s.checkOverlap(cd.allTables(), cd.nextLevel.level+1)

y/metrics.go

@@ -48,6 +48,8 @@ var (
 	NumBlockedPuts *expvar.Int
 	// NumMemtableGets is number of memtable gets
 	NumMemtableGets *expvar.Int
+	// NumCompactionTables is the number of tables being compacted
+	NumCompactionTables *expvar.Int
 )
 
 // These variables are global and have cumulative values for all kv stores.
@@ -65,4 +67,5 @@ func init() {
 	LSMSize = expvar.NewMap("badger_v2_lsm_size_bytes")
 	VlogSize = expvar.NewMap("badger_v2_vlog_size_bytes")
 	PendingWrites = expvar.NewMap("badger_v2_pending_writes_total")
+	NumCompactionTables = expvar.NewInt("badger_v2_compactions_current")
 }