[UX] Add --list-metrics option to list all available metrics

[xuzhao9]

Add --list-metrics to list all available metrics.

Add --list-backends to list all backends and their tags, we should be able to tag PT2, Triton, and CUTLASS backends.

[adamomainz]

what would --list-metrics provide here? a list of all possible metrics that come standard or a list of metrics for a specific op? same for backends

[FindHao]

currently, we have different kinds of metrics:

• common ones defined in triton_op.py. they are general to the whole tests.
• metrics defined in components, for now ncu/nsys analyzer have its own metric list. specific for different components.
• metrics defined in individual ops. ops use register_metric to register their own metrics. specific for ops.
• metrics defined in operator loader . a different component from operators.