cmd/stdiscosrv: Expose process metrics

Like this:

    $ curl -s http://localhost:9098/metrics | egrep '^syncthing_discovery_process'
    syncthing_discovery_process_cpu_seconds_total 12.92
    syncthing_discovery_process_max_fds 10240
    syncthing_discovery_process_open_fds 51
    syncthing_discovery_process_resident_memory_bytes 1.3674496e+08
    syncthing_discovery_process_start_time_seconds 1.52034731837e+09
    syncthing_discovery_process_virtual_memory_bytes 1.40324864e+08

cmd/stdiscosrv/stats.go

@@ -7,6 +7,8 @@
 package main
 
 import (
+	"os"
+
 	"github.com/prometheus/client_golang/prometheus"
 )
 
@@ -105,4 +107,6 @@ func init() {
 		replicationSendsTotal, replicationRecvsTotal,
 		databaseKeys, databaseStatisticsSeconds,
 		databaseOperations, databaseOperationSeconds)
+
+	prometheus.MustRegister(prometheus.NewProcessCollector(os.Getpid(), "syncthing_discovery"))
 }