Add user tag to procstat input (#4386)

plugins/inputs/procstat/process.go

@@ -21,6 +21,7 @@ type Process interface {
 	Percent(interval time.Duration) (float64, error)
 	Times() (*cpu.TimesStat, error)
 	RlimitUsage(bool) ([]process.RlimitStat, error)
+	Username() (string, error)
 }
 
 type PIDFinder interface {
@@ -58,6 +59,10 @@ func (p *Proc) PID() PID {
 	return PID(p.Process.Pid)
 }
 
+func (p *Proc) Username() (string, error) {
+	return p.Process.Username()
+}
+
 func (p *Proc) Percent(interval time.Duration) (float64, error) {
 	cpu_perc, err := p.Process.Percent(time.Duration(0))
 	if !p.hasCPUTimes && err == nil {