Is your feature request related to a problem?

Could we use add ENABLE_METRICS_LEGACY instead of using ENABLE_METRICS_PREVIEW to decide whether to use legacy metrics SDK and exporter?

The legacy metrics depends RTTI, by use a new option, we can enable new metrics without RTTI.

Describe the solution you'd like

Add ENABLE_METRICS_LEGACY and replace all checking in _metrics.

Additional context

If this is acceptable, it can be assigned to me.