[LTC] Refine MetricsArena::Reset (#89608)
Summary:
After counters are reset, getters' behaviors are inconsistent. To improve that, here I 1) move the validation of CounterData into CounterData::IsValid such that it's better encapsulated, 2) divide getters into two groups: a) MetricsArena::GetCounter() and b) MetricsArena::ForEachCounter(), and route MetricsArena::GetCounterNames() and CreateMetricReport() to use b.
This is paired with pytorch/xla#4217.
Test Plan:
PJRT_DEVICE=CPU python xla/test/test_metrics.py
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89608
Approved by: https://github.com/JackCaoG
