Fix metric description of prometheus_tsdb_symbol_table_size_bytes. (#8080)

This is how much memory we use to load in the on-disk
symbol tables, not the size of the tables themselves.

Signed-off-by: Brian Brazil <brian.brazil@robustperception.io>

tsdb/db.go

@@ -199,7 +199,7 @@ func newDBMetrics(db *DB, r prometheus.Registerer) *dbMetrics {
 	})
 	m.symbolTableSize = prometheus.NewGaugeFunc(prometheus.GaugeOpts{
 		Name: "prometheus_tsdb_symbol_table_size_bytes",
-		Help: "Size of symbol table on disk (in bytes)",
+		Help: "Size of symbol table in memory for loaded blocks",
 	}, func() float64 {
 		db.mtx.RLock()
 		blocks := db.blocks[:]