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?