Clarify Rust logging behavior in samples

[martijneken]

All our Rust plugin samples have this line, with no explanation:
proxy_wasm::set_log_level(LogLevel::Trace);

This is extra confusing because Service Extensions has its own min_log_level config which applies to all wasm logging.

Task: use code comments to clarify why the code is needed.

The code is needed because:

• Rust has its own max_log_level, and it defaults all logging to OFF: https://docs.rs/log/latest/log/#implementing-a-logger
• the Rust SDK implementation does not change this default: https://github.com/proxy-wasm/proxy-wasm-rust-sdk/blob/master/src/logger.rs

[martijneken]

Consider also updating the Rust SDK to set a non-off log level default.