Feature | Emit OpenTelemetry metrics

OpenTelemetry has become the de-facto standard for distributed, cross-platform tracing; it is being rapidly adopted both inside the .NET ecosystem and outside. OTel defines semantic conventions for how a database client emits metrics, which are aggregated, numeric counters about the state and performance of the driver (e.g. number of commands executed, idle/busy connections in the pool, etc.). [these specifications](https://opentelemetry.io/docs/specs/semconv/database/database-metrics/) are currently in experimental state, but stabilization is likely start happening soon.

Note that unlike with [tracing](https://github.com/dotnet/SqlClient/issues/2210), AFAIK there's no way to get data like this out of SqlClient at the moment. This sort of data can be invaluable in diagnosing performance problems (e.g. by inspecting real-time connection pool state) and tracking what's going on.