fix(remotewrite2): do not overwrite help text with unit in enqueue

While testing POC for https://github.com/grafana/mimir/issues/9072
I saw no unit or help metadata. Our test env:
https://github.com/grafana/mimir/tree/main/development/mimir-monolithic-mode
doesn't have units, so that was empty and cleared the help due to this bug.

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>

storage/remote/queue_manager.go

@@ -1923,7 +1923,7 @@ func populateV2TimeSeries(symbolTable *writev2.SymbolsTable, batch []timeSeries,
 		if d.metadata != nil {
 			pendingData[nPending].Metadata.Type = writev2.FromMetadataType(d.metadata.Type)
 			pendingData[nPending].Metadata.HelpRef = symbolTable.Symbolize(d.metadata.Help)
-			pendingData[nPending].Metadata.HelpRef = symbolTable.Symbolize(d.metadata.Unit)
+			pendingData[nPending].Metadata.UnitRef = symbolTable.Symbolize(d.metadata.Unit)
 			nPendingMetadata++
 		}