[Feature] Basic systemd service monitoring (#1153)

* basic systemd service monitoring

* update to work after /internal rename

* monitor systemd service cpu and memory usage

---------

Co-authored-by: henrygd <hank@henrygd.me>

internal/entities/systemd/systemd.go

@@ -0,0 +1,34 @@
+package systemd
+
+import (
+	"runtime"
+	"time"
+)
+
+// Service represents a single systemd service with its stats.
+type Service struct {
+	Name         string    `json:"n" cbor:"0,keyasint"`
+	Status       string    `json:"s" cbor:"1,keyasint"`
+	Cpu          float64   `json:"c" cbor:"2,keyasint"`
+	Mem          float64   `json:"m" cbor:"3,keyasint"`
+	PrevCpuUsage uint64    `json:"-"`
+	PrevReadTime time.Time `json:"-"`
+}
+
+// CalculateCPUPercent calculates the CPU usage percentage for the service.
+func (s *Service) CalculateCPUPercent(cpuUsage uint64) {
+	if s.PrevReadTime.IsZero() {
+		s.Cpu = 0
+	} else {
+		duration := time.Since(s.PrevReadTime).Nanoseconds()
+		if duration > 0 {
+			coreCount := int64(runtime.NumCPU())
+			duration *= coreCount
+			cpuPercent := float64(cpuUsage-s.PrevCpuUsage) / float64(duration)
+			s.Cpu = cpuPercent * 100
+		}
+	}
+
+	s.PrevCpuUsage = cpuUsage
+	s.PrevReadTime = time.Now()
+}