Receive: fix thanos_receive_write_{timeseries,samples} stats

There are two path a data can be written to a receiver: through the HTTP
or the gRPC endpoint, and `thanos_receive_write_{timeseries,samples}` only
count the number of timeseries/samples received through the HTTP
endpoint.

So, there is no risk that a sample will be count twice, once as a
remote write and once as a local write. On the other hand, we still need
to account for the replication factor, and only count local writes is
not enough as there might be no local writes at all (e.g. in RouterOnly
mode).

pkg/receive/handler.go

@@ -683,8 +683,11 @@ type remoteWriteParams struct {
 
 func (h *Handler) gatherWriteStats(writes ...map[endpointReplica]map[string]trackedSeries) tenantRequestStats {
 	var stats tenantRequestStats = make(tenantRequestStats)
+	var rf int = 0
 
 	for _, write := range writes {
+		rf += len(write)
+
 		for er := range write {
 			for tenant, series := range write[er] {
 				samples := 0
@@ -708,8 +711,13 @@ func (h *Handler) gatherWriteStats(writes ...map[endpointReplica]map[string]trac
 		}
 	}
 
-	return stats
+	// adjust samples counters by the replication factor
+	for tenant, st := range stats {
+		st.totalSamples /= rf
+		stats[tenant] = st
+	}
 
+	return stats
 }
 
 func (h *Handler) fanoutForward(ctx context.Context, params remoteWriteParams) (tenantRequestStats, error) {