Added explanatory comments for RuntimeMetrics methods

Motivation

The current documentation for RuntimeMetrics methods doesn't clearly indicate when both tokio_unstable and 64-bit atomic features are required. This can lead to confusion for users trying to use these metrics, especially when compiling for different targets or with different feature sets.

Solution

As a temporary measure, I've added inline comments to the RuntimeMetrics implementation in runtime.rs. These comments explicitly state when a method requires both the tokio_unstable feature and 64-bit atomics.

This is intended as an intermediate step towards a more comprehensive solution that may involve modifying the cfg macros to properly combine these conditions in the documentation.

Changes made:

• Added comments above relevant methods in RuntimeMetrics implementation in runtime.rs
• Comments clearly state when both tokio_unstable and 64-bit atomics are required
• No functional changes to the code itself
  Fixes Some unstable runtime metrics show up as stable in docs.rs #6745