collab: Attach User-Agent to handle connection span (#35282)

This PR makes it so we attach the value from the `User-Agent` header to
the `handle connection` span.

We'll start sending this header in
https://github.com/zed-industries/zed/pull/35280.

Release Notes:

- N/A

crates/collab/src/tests/test_server.rs

@@ -256,6 +256,7 @@ impl TestServer {
                             ZedVersion(SemanticVersion::new(1, 0, 0)),
                             None,
                             None,
+                            None,
                             Some(connection_id_tx),
                             Executor::Deterministic(cx.background_executor().clone()),
                             None,