Tokio has an unstable feature supporting runtime metrics.
This allows service operators to export metrics to systems like
Prometheus, so that they can observe how their workloads are performing
on the tokio runtime. This exposes information like the number of worker
threads, queue depth, the number of tasks polled, and so on.
`linkerd2-proxy` should expose these metrics.
This uses the `kubert-prometheus-tokio` crate to register a `Runtime`
metrics worker, and spawn a task to probe these metrics at a fixed,
regular interval.
see: <https://github.com/olix0r/kubert/tree/main/kubert-prometheus-tokio>
if the `tokio_unstable` feature is not enabled, this will emit a debug
event and do nothing.
Signed-off-by: katelyn martin <kate@buoyant.io>
193dcaf818