Add cmdline to tracing output

Can come in handy when other people report problems and show the error
message or full log, but not the command line.

Setting that span to `level = "error"` ensures that it won't be disabled
by level filters.

crates/dtmt/src/main.rs

@@ -38,10 +38,21 @@ struct GlobalConfig {
 }
 
 #[tokio::main]
-#[tracing::instrument]
+#[tracing::instrument(level = "error", fields(cmd_line = tracing::field::Empty))]
 async fn main() -> Result<()> {
     color_eyre::install()?;
 
+    {
+        let span = tracing::Span::current();
+        if !span.is_disabled() {
+            let cmdline: String = std::env::args_os().fold(String::new(), |mut s, arg| {
+                s.push_str(&arg.to_string_lossy());
+                s
+            });
+            span.record("cmd_line", cmdline);
+        }
+    }
+
     let matches = command!()
         .subcommand_required(true)
         .arg(