Add tokio runtime metrics

[mzacho]

Self-explanatory.

Tested locally in kind, the new metrics as reported from localhost:15020/metrics:

# HELP tokio_num_workers the number of worker threads used by the runtime
# TYPE tokio_num_workers gauge
tokio_num_workers 2
# HELP tokio_global_queue_depth the number of tasks currently scheduled in the runtime's global queue
# TYPE tokio_global_queue_depth gauge
tokio_global_queue_depth 0
# HELP tokio_num_alive_tasks the number of alive tasks in the runtime
# TYPE tokio_num_alive_tasks gauge
tokio_num_alive_tasks 4
# HELP tokio_worker_total_busy_duration_seconds the amount of time worker threads have been busy
# TYPE tokio_worker_total_busy_duration_seconds counter
tokio_worker_total_busy_duration_seconds_total{worker="0"} 0.000133915
tokio_worker_total_busy_duration_seconds_total{worker="1"} 0.002096876
# HELP tokio_worker_park_count the total number of times the given worker thread has parked
# TYPE tokio_worker_park_count counter
tokio_worker_park_count{worker="0"} 9
tokio_worker_park_count{worker="1"} 11
# HELP tokio_worker_park_unpark_count the total number of times the given worker thread has parked and unparked
# TYPE tokio_worker_park_unpark_count counter
tokio_worker_park_unpark_count{worker="0"} 17
tokio_worker_park_unpark_count{worker="1"} 21

[istio-testing]

Hi @mzacho. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[keithmattix]

/ok-to-test

[mzacho]

make check-features failed when doing cd fuzz; RUSTFLAGS="--cfg fuzzing" cargo check since it's compiling ztunnel with tokio version 1.44.2 where the metrics worker_park_count, worker_total_busy_duration and worker_park_unpark_count weren't stabilized yet.

Ran cd fuzz && cargo update to update fuzz/Cargo.lock which fixes this.

[mzacho]

/retest