fix(plugins/metric_system_v2): report fd metrics as string to avoid float64 precision loss

[zhang-guodong]

Hi maintainers,

I noticed that input plugin metric_system_v2 reports fd_max as float64. However, fd_max is fixed at max int64(2^63-1 or 9223372036854775807), which exceeds the range of float64. Therefore, the data, with its loss of precision, is represented as 9.223372036854776e+18 in Metric.

loongcollector/plugins/input/systemv2/input_system_linux.go

Lines 166 to 169 in 68ba314

	allocated, _ := strconv.ParseFloat(string(parts[0]), 64)
	maximum, _ := strconv.ParseFloat(string(parts[2]), 64)
	r.addMetric(collector, "fd_allocated", &r.commonLabels, allocated)
	r.addMetric(collector, "fd_max", &r.commonLabels, maximum)

Since the metric is eventually converted to a string (see log_helper.go#L258), it would be more accurate to record fd_max directly as a string.

Alternatively, how about considering simply removing fd_max, since it's a constant on Linux anyway?

[Takuka0311]

百炼自动化审查：建议保持开启。

本 PR 修复了 metric_system_v2 插件中 fd 指标因 float64 转换导致的大整数精度丢失问题，改为使用字符串上报。经核对当前 main 分支，该修复尚未合入，且无其他替代实现或重复 PR，属于有效贡献，应保持开启供维护者审查。.

最佳落地路径：

建议保持开启并等待维护者审查。需重点验证下游指标消费端对字符串类型指标值的兼容性。若确认 helper.NewMetricLogStringVal 接口符合规范且无破坏性变更，可在 CI 通过后合并；或根据 PR 正文建议评估是否直接移除恒定的 fd_max 指标以简化逻辑。.

已核对内容：

• main_state_check: 当前 main 分支仍使用 strconv.ParseFloat 处理 fd_allocated 与 fd_max 指标，精度丢失问题依然存在，本 PR 修复尚未合入。. (plugins/input/systemv2/input_system_linux.go:166, 7099f790b8a3)
• pr_diff_analysis: PR 移除 ParseFloat 调用，新增 addMetricStringVal 方法直接上报字符串，改动最小化且逻辑正确。. (plugins/input/systemv2/input_system_linux.go:162, 2f363a2cffd3)
• api_compatibility: helper 包已提供 NewMetricLogStringVal 接口支持字符串指标值，PR 方案具备底层实现基础。. (pkg/helper/log_helper.go:258, 7099f790b8a3)

百炼审查备注：模型 qwen3.6-max-preview；对照提交 7099f790b8a3。