Fix NFSd metric type (#819)

RPC Count should be a counter, not a gauge.

collector/fixtures/e2e-output.txt

@@ -2686,7 +2686,7 @@ node_nfsd_rpc_errors_total{error="auth"} 2
 node_nfsd_rpc_errors_total{error="cInt"} 0
 node_nfsd_rpc_errors_total{error="fmt"} 1
 # HELP node_nfsd_server_rpcs_total Total number of NFSd RPCs.
-# TYPE node_nfsd_server_rpcs_total gauge
+# TYPE node_nfsd_server_rpcs_total counter
 node_nfsd_server_rpcs_total 18628
 # HELP node_nfsd_server_threads Total number of NFSd kernel threads that are running.
 # TYPE node_nfsd_server_threads gauge

collector/nfsd_linux.go

@@ -230,7 +230,7 @@ func (c *nfsdCollector) updateNFSdServerRPCStats(ch chan<- prometheus.Metric, s
 			nil,
 			nil,
 		),
-		prometheus.GaugeValue,
+		prometheus.CounterValue,
 		float64(s.RPCCount))
 }