Expose runtime metrics

[olix0r]

When building applications with tokio, it can be hard to understand reactor/executor behavior at runtime.

I want to be able to ask a few questions of the runtime like the following:

• How many tasks have been scheduled? [total since runtime creation]
• How many tasks are currently scheduled? [instantaneous]
• How much time has been sent polling tasks? [total since runtime creation]

It's important that these values are not time-bounded or otherwise pre-rated so that they fit well into timeseries -- by tracking totals we can determine the rate at read-time without having to configure the runtime with a window...