Add some tokio runtime metrics.

[jsdt]

Description of Changes

Adding some tokio runtime stats. Metrics like tokio_num_alive_tasks and tokio_global_queue_depth could be useful to see if tokio is falling behind. Tokio metrics are described here.

There are some other per-worker stats that could be useful to add in the future if we are having issues with tokio scheduling.

Many of the metrics require using the tokio_unstable compilation flag, which is awkward to enable. The code will still compile if tokio_unstable is not enabled, though it will have some unused variable warnings.

API and ABI breaking changes

If this is an API or ABI breaking change, please apply the
corresponding GitHub label.

Expected complexity level and risk

Testing

Tested by looking at metrics locally.

[Copilot]

Pull Request Overview

This PR adds several new metrics to monitor the Tokio runtime, providing insights into worker utilization and task scheduling.

• Added new metric definitions for various Tokio runtime parameters (worker count, blocking threads, task counts, and queue depths).
• Implemented the spawn_tokio_stats function to collect and update these metrics periodically.
• Updated the Cargo configuration to enable the tokio_unstable flag required for these metrics.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
crates/core/src/worker_metrics/mod.rs	Introduces new Tokio runtime metrics and the stats collection task
.cargo/config.toml	Configures the build to use the tokio_unstable flag