Observe SDK: limit spans sent to sink

- [ ] Go SDK
- [ ] Rust SDK
- [ ] JS SDK

The default should be applied similarly to each SDK, but have a method for configuring this at the SDK level.

For example, a possible approach is:
```rust
let mut trace_ctx = adapter.start(&mut linker, &data)?;
trace_ctx.set_trace_depth(4);

// OR.. avoiding mutability and any additional method surface area:

let config = TraceConfig {
trace_depth: 4,
..default::Default(),
};
let trace_ctx = adapter.start(&mut linker, &data, &config)?;
```