[SVLS-8351] Add CPU Enhanced Metrics

[kathiehuang]

What does this PR do?

• Adds CPU limit and rate enhanced metrics
  • Creates a metrics collector that reads and submit CPU metrics every 3 seconds
  • Sets up:
    • CgroupStats struct for reading statistics from cgroup v1 files
      • This normalizes the stats to nanoseconds
    • CpuStats struct to store the computed CPU total and limit metrics
      • Converts u64 values to to f64
      • Calculates CPU limit percentage
    • CpuMetricsCollector struct that will collect this information at an interval and submit as metrics

Motivation

Additional Notes

Describe how to test/QA your changes

Built with serverless-compat-self-monitoring.

To do: Test different collection and aggregation intervals to find optimal balance between collection frequency and added overhead

Added a bunch of debug logs to see what was going on with the calculations:

DEBUG datadog_trace_agent::metrics_collector: Contents of /sys/fs/cgroup/cpuset/cpuset.cpus: 0-1
DEBUG datadog_trace_agent::metrics_collector: Range: ["0", "1"]
DEBUG datadog_trace_agent::metrics_collector: Total CPU count: 2
DEBUG datadog_trace_agent::metrics_collector: CFS scheduler quota is -1, setting to None
DEBUG datadog_trace_agent::metrics_collector: Could not read scheduler quota from /sys/fs/cgroup/cpu/cpu.cfs_quota_us
DEBUG datadog_trace_agent::metrics_collector: No CPU limit found, defaulting to host CPU count: 2 CPUs
DEBUG datadog_trace_agent::metrics_collector: Collected cpu stats!
DEBUG datadog_trace_agent::metrics_collector: CPU usage: 9871234519
DEBUG datadog_trace_agent::metrics_collector: CPU limit: 200%, defaulted: true
DEBUG datadog_trace_agent::metrics_collector: Submitting CPU metrics!

Looks good but can't read scheduler quota from cpu.cfs_quota_us, so it always falls back to the host cpu count from the crate num_cpus

References: datadog-agent cgroup collection and calculation logic