db: add method to get remote table size, count

This patch adds a method to our `Metrics` struct, `RemoteTablesTotal`,
that will compute the total size and count of remote tables.

Informs: #143850

Author: annrpom

metrics.go

@@ -443,6 +443,26 @@ func (m *Metrics) Total() LevelMetrics {
 	return total
 }
 
+// RemoteTablesTotal returns the total number of remote tables and their total
+// size. Remote tables are computed as the difference between total tables
+// (live + obsolete + zombie) and local tables.
+func (m *Metrics) RemoteTablesTotal() (count uint64, size uint64) {
+	var liveTables, liveTableBytes int64
+	for level := 0; level < numLevels; level++ {
+		liveTables += m.Levels[level].NumFiles
+		liveTableBytes += m.Levels[level].Size
+	}
+	totalCount := liveTables + m.Table.ObsoleteCount + m.Table.ZombieCount
+	localCount := m.Table.Local.LiveCount + m.Table.Local.ObsoleteCount + m.Table.Local.ZombieCount
+	remoteCount := uint64(totalCount) - localCount
+
+	totalSize := uint64(liveTableBytes) + m.Table.ObsoleteSize + m.Table.ZombieSize
+	localSize := m.Table.Local.LiveSize + m.Table.Local.ObsoleteSize + m.Table.Local.ZombieSize
+	remoteSize := totalSize - localSize
+
+	return remoteCount, remoteSize
+}
+
 // String pretty-prints the metrics as below:
 //
 //	      |                             |       |       |   ingested   |     moved    |    written   |       |    amp

metrics_test.go

@@ -395,6 +395,12 @@ func TestMetrics(t *testing.T) {
 						panic(fmt.Sprintf("invalid level %d", l))
 					}
 					buf.WriteString(fmt.Sprintf("%d\n", m.Levels[l].NumVirtualFiles))
+				} else if line == "remote-count" {
+					count, _ := m.RemoteTablesTotal()
+					buf.WriteString(fmt.Sprintf("%d\n", count))
+				} else if line == "remote-size" {
+					_, size := m.RemoteTablesTotal()
+					buf.WriteString(fmt.Sprintf("%s\n", humanize.Bytes.Uint64(size)))
 				} else {
 					panic(fmt.Sprintf("invalid field: %s", line))
 				}
@@ -427,6 +433,7 @@ func TestMetrics(t *testing.T) {
 			return fmt.Sprintf("unknown command: %s", td.Cmd)
 		}
 	})
+
 }
 
 func TestMetricsWAmpDisableWAL(t *testing.T) {

testdata/metrics

@@ -870,6 +870,13 @@ set b 2
 ingest ext1.sst
 ----
 
+metrics-value
+remote-count
+remote-size
+----
+2
+1.5KB
+
 lsm
 ----
 L0.0: