profile: Add instrumentation to startup related functions (#44456)

Follow-up to #44443.
This helps investigating the cold-start timeline, and could be used
by tooling to A/B compare branches affecting the cold-start time.

Additionally also change the `handle_request::select` span, so that we
can see the blocked time (which was probably what was intended), since
the actual time spent on recv after select is insignificant.

Testing: Tracing output is not covered by automatic tests.

---------

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

ports/servoshell/desktop/app.rs

@@ -129,6 +129,7 @@ impl App {
         self.state = AppState::Running(running_state);
     }
 
+    #[servo::servo_tracing::instrument(level = "debug", skip_all)]
     fn create_platform_window(
         &self,
         url: Url,

ports/servoshell/desktop/headed_window.rs

@@ -105,6 +105,7 @@ pub struct HeadedWindow {
 }
 
 impl HeadedWindow {
+    #[servo::servo_tracing::instrument(level = "debug", name = "HeadedWindow::new", skip_all)]
     pub(crate) fn new(
         servoshell_preferences: &ServoShellPreferences,
         event_loop: &ActiveEventLoop,

ports/servoshell/desktop/headless_window.rs

@@ -35,6 +35,7 @@ pub struct HeadlessWindow {
 }
 
 impl HeadlessWindow {
+    #[servo::servo_tracing::instrument(level = "debug", name = "HeadlessWindow::new", skip_all)]
     pub fn new(servoshell_preferences: &ServoShellPreferences) -> Rc<Self> {
         let size = servoshell_preferences.initial_window_size;
 

ports/servoshell/egl/app.rs

@@ -277,6 +277,7 @@ pub struct App {
 
 #[expect(unused)]
 impl App {
+    #[servo::servo_tracing::instrument(skip_all, name = "App::new", level = "info")]
     pub(super) fn new(init: AppInitOptions) -> Rc<Self> {
         let mut servo_builder = ServoBuilder::default()
             .opts(init.opts)

ports/servoshell/window.rs

@@ -86,6 +86,7 @@ impl ServoShellWindow {
     }
 
     /// Must be called *after* `self` is in `state.windows`, otherwise it will panic.
+    #[servo::servo_tracing::instrument(skip(self, state))]
     pub(crate) fn create_toplevel_webview(&self, state: Rc<RunningAppState>, url: Url) -> WebView {
         let mut webview_builder =
             WebViewBuilder::new(state.servo(), self.platform_window.rendering_context())