PrometheusBuilder::install: Build exporter in runtime context

Closes metrics-rs#122

metrics-exporter-prometheus/src/lib.rs

@@ -443,14 +443,14 @@ impl PrometheusBuilder {
     /// installing the recorder as the global recorder.
     #[cfg(feature = "tokio-exporter")]
     pub fn install(self) -> Result<(), Error> {
-        let (recorder, exporter) = self.build_with_exporter()?;
-        metrics::set_boxed_recorder(Box::new(recorder))?;
-
         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))?;
+
         thread::Builder::new()
             .name("metrics-exporter-prometheus-http".to_string())
             .spawn(move || {