single-thread tokio

not actually sure if this is needed for the raspi but initially i thought metrics weren't working. (they are fine)

who-am-i/src/main.rs

···
 use clap::{ArgAction, Parser};
-use metrics_exporter_prometheus::PrometheusBuilder;
+use metrics_exporter_prometheus::{PrometheusBuilder, BuildError as PromBuildError};
 use tokio_util::sync::CancellationToken;
 use who_am_i::serve;
 
···
     allowed_hosts: Vec<String>,
 }
 
-#[tokio::main]
+#[tokio::main(flavor = "current_thread")]
 async fn main() {
     let shutdown = CancellationToken::new();
 
···
     serve(shutdown, args.app_secret, args.allowed_hosts, args.dev).await;
 }
 
-fn install_metrics_server() -> Result<(), metrics_exporter_prometheus::BuildError> {
+fn install_metrics_server() -> Result<(), PromBuildError> {
     println!("installing metrics server...");
     let host = [0, 0, 0, 0];
     let port = 8765;