meta: instrument Tokio with tracing.

[carllerche]

This issue tracks instrumenting Tokio using the tracing crate (formerly known as tokio-trace). I will be updating the description with thoughts and the issue can be used for general discusssion.

Runtime

• Active tasks (Task counter for Runtime #697).
• Track worker-id polling task (Question: Determining which worker ID that a task is running on #815).

Net

• net: expose consistent tracing events #1504

[hawkw]

I opened #1504 to discuss exposing more diagnostic events from tokio-net.

[hawkw]

#2655 adds tracing spans for every spawned task, when enabled. We can derive information about task durations (and counting different types of task) using these spans.

[carllerche]

Gonna close this for now as it hasn't took off. We do want more instrumentation points though :)