Add resource_slots_cpu/mem_usage metrics

[yuandrew]

What was changed

Add resource_slots_cpu_usage and resource_slots_mem_usage metrics

Why?

Match Core and Java

Checklist

1. Closes Add resource tuner metrics #2089

2. How was this tested:
   Added a test

3. Any docs updates needed?
   Should update https://docs.temporal.io/references/sdk-metrics#resource_slots_cpu_usage to include Go in the list of supported SDKs

---

Note

Adds resource_slots_cpu_usage and resource_slots_mem_usage gauges and emits them during PID decisions in the resource tuner, with accompanying tests.

• Metrics:
  • Add constants metrics.ResourceSlotsCPUUsage and metrics.ResourceSlotsMemUsage.
• Resource Tuner (contrib/resourcetuner):
  • Plumb metrics.Handler into pidDecision and TryReserveSlot to publish usage.
  • Implement publishResourceMetrics to update gauges with percent values.
• Tests:
  • Update existing tests to pass a metrics handler.
  • Add test TestPidDecisionEmitsUsageMetrics verifying both gauges and values.

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[Quinn-With-Two-Ns]

Don't think we should be using internal in a external package

[Quinn-With-Two-Ns]

TBH I am surprised this isn't failing a lint error