PrometheusBuilder::install panics without a thread-local runtime

[str4d]

PrometheusBuilder::install calls PrometheusBuilder::build_with_exporter to configure the exporter, then creates a tokio runtime and uses it to run the exporter.

In #92 PrometheusBuilder::build (now build_with_exporter) was refactored to call Server::try_bind before starting the server. This means that hyper operations happen outside the exporter future, whereas previously all hyper operations happened inside Runtime::block_on.

The problem with this is that Server::try_bind eventually calls tokio::net::tcp::TcpListener::from_std, which panics if thread-local runtime is not set:

    /// # Panics
    ///
    /// This function panics if thread-local runtime is not set.
    ///
    /// The runtime is usually set implicitly when this function is called
    /// from a future driven by a tokio runtime, otherwise runtime can be set
    /// explicitly with [`Handle::enter`](crate::runtime::Handle::enter) function.

I believe the solution is to create the runtime first, and then enter its context when building the exporter:

    pub fn install(self) -> Result<(), Error> {
        let mut runtime = runtime::Builder::new()
            .basic_scheduler()
            .enable_all()
            .build()?;

        let (recorder, exporter) = runtime.enter(|| self.build_with_exporter())?;
        metrics::set_boxed_recorder(Box::new(recorder))?;