fix(agent): add fallback for CPU model detection on MIPS architectures (#2138)

gopsutil's cpu.Info() does not parse the 'cpu model' field from
/proc/cpuinfo, which is the only source of CPU model names on MIPS.
Add a fallback that reads /proc/cpuinfo directly and combines
'cpu model' (e.g. 'MIPS 1004Kc V2.15') with 'system type'
(e.g. 'MediaTek MT7621 ver:1 eco:3') for a complete identifier.

The fallback only triggers when gopsutil returns an empty ModelName,
so x86/ARM/other architectures are unaffected.

agent/fingerprint.go

@@ -50,6 +50,9 @@ func generateFingerprint(hostname, cpuModel string) string {
 			if info, err := cpu.Info(); err == nil && len(info) > 0 {
 				cpuModel = info[0].ModelName
 			}
+			if cpuModel == "" {
+				cpuModel = getCpuModelFromCpuinfo()
+			}
 		}
 		fingerprint = hostname + cpuModel
 	}