[metrics] Optimize lookup costs using .NET 9 alternative lookup

[cijothomas]

Metrics hot path now involves copying the incoming ReadOnlySpan into a Thread-Local array to do the dictionary lookup. With the new capability to lookup Dictionary with just ReadOnlySpan, we can avoid the thread-local copy cost. Opening an issue to track this.

Unrelated/Related : Rust's Dictionary(hashmap) already allowed such lookups, and I leveraged that in OTel Rust metrics and avoided entire copy costs and got good per gains (open-telemetry/opentelemetry-rust#1989).