Push doesn't work inside tokio async block

[adamchalmers]

Describe the bug
The prometheus::push_metrics function panics if called inside an async block (at least when using the Tokio runtime). It panics with the message 'Cannot drop a runtime in a context where blocking is not allowed. This happens when a runtime is dropped from within an asynchronous context.'

I believe this is due to the use of reqwest::blocking::Client in the implementation. One backwards-compatible solution would be adding a new prometheus::push_metrics_async function that uses reqwest::Client instead. I would be happy to open a PR to add this, if you like the suggestion.

To Reproduce
Minimal example to reproduce the behavior:

src/main.rs

#[macro_use]
extern crate prometheus;
use prometheus::{gather, push_metrics, Error};
use std::collections::HashMap;

#[tokio::main]
async fn main() -> Result<(), Error> {
    let counter = register_counter!("example_counter", "Just a counter").unwrap();
    counter.inc();
    push_metrics("example", HashMap::new(), "localhost:9091", gather(), None)?;
    println!("Success");
    Ok(())
}

Cargo.toml

[package]
name = "prometheus-bug"
version = "0.1.0"
authors = ["Adam Chalmers <adam.s.chalmers@gmail.com>"]
edition = "2018"

[dependencies]
tokio = { version = "0.2.22", features = ["macros"] }

[dependencies.prometheus]
version = "0.10"
default-features = true
features = ["push", "process"]

Expected behavior
push_metrics() does not panic when called inside an async function using Tokio.

System information

• CPU architecture: Intel i7
• Distribution and kernel version: MacOS Mojave

[adamchalmers]

Actually, this is fixed in Tokio 1.0, changing the Cargo.toml from the example to use

tokio = { version = "1", features = ["macros", "time", "rt-multi-thread"] }

will cause the program to terminate successfully.