fix(agent): read /proc/uptime on linux instead of sysinfo(2) (#2180)

gopsutil's host.Uptime() calls the sysinfo(2) syscall. Inside an LXC
container lxcfs virtualizes /proc/uptime but cannot intercept a
syscall, so every container reported the host's uptime.

Reads /proc/uptime on linux and falls back to host.Uptime() if the file
is missing or unparseable, so other platforms are unchanged.

agent/uptime_linux.go

@@ -0,0 +1,44 @@
+//go:build linux
+
+package agent
+
+import (
+	"math"
+	"os"
+	"strconv"
+	"strings"
+
+	"github.com/shirou/gopsutil/v4/host"
+)
+
+// uptimeFilePath is a variable so tests can point it at a fixture.
+var uptimeFilePath = "/proc/uptime"
+
+// getUptime returns the system uptime in seconds.
+//
+// This reads /proc/uptime instead of using host.Uptime(), which calls the
+// sysinfo(2) syscall. Inside an LXC container lxcfs virtualizes /proc/uptime
+// but cannot intercept a syscall, so sysinfo(2) reports the host's uptime
+// rather than the container's.
+//
+// Falls back to host.Uptime() if /proc/uptime is missing or unparseable, so
+// behavior is unchanged anywhere the file isn't available.
+func getUptime() (uint64, error) {
+	data, err := os.ReadFile(uptimeFilePath)
+	if err != nil {
+		return host.Uptime()
+	}
+	fields := strings.Fields(string(data))
+	if len(fields) == 0 {
+		return host.Uptime()
+	}
+	seconds, err := strconv.ParseFloat(fields[0], 64)
+	if err != nil ||
+		math.IsNaN(seconds) ||
+		math.IsInf(seconds, 0) ||
+		seconds < 0 ||
+		seconds >= 1<<64 {
+		return host.Uptime()
+	}
+	return uint64(seconds), nil
+}