utils/prometheus: Make crate spawn onto global executor

[mxinden]

With 6ee1244 the Substrate green threading executor is now
configurable. The Substrate Prometheus crate can use this executor to
spawn futures per connection (scrape request).

Everything other than the last commit (ea9278a) can be ignored as it just merges current Substrate master.

[expenses]

Looks good. Not a blocking change, but does it work if you change the task executor to:

config.tasks_executor = {
	Some(Box::new(move |fut| { async_std::task::spawn(fut); }))
};

?

[nodebreaker0-0]

I don't understand deeply how global-executor behavior should be written in substrate.
I will follow your choice.

[mxinden]

Closing here for now, given that I am not sure how to progress:

• We can use the global executor, but once the user configures it to be async-std hyper will panic, given that it is not compatible by default.

• We could spawn a separate Tokio runtime on a new thread.

• We could keep it as is, which has much in common to the approach above, except that we would use async-std.