Add tracing for navigation start, FP, FCP, and DOMContentLoaded (#34373)as-of-2024-08-12

Signed-off-by: Delan Azabani <dazabani@igalia.com>

2 files changed, 30 insertions, 11 deletions

diff --git a/components/metrics/Cargo.toml b/components/metrics/Cargo.toml
index f2464ccf24b..7f12c50ccf4 100644
--- a/components/metrics/Cargo.toml
+++ b/components/metrics/Cargo.toml
@@ -10,6 +10,9 @@ publish.workspace = true
 name = "metrics"
 path = "lib.rs"
 
+[features]
+tracing = ["dep:tracing"]
+
 [dependencies]
 base = { workspace = true }
 fonts_traits = { workspace = true }
@@ -21,3 +24,4 @@ profile_traits = { workspace = true }
 script_traits = { workspace = true }
 servo_config = { path = "../config" }
 servo_url = { path = "../url" }
+tracing = { workspace = true, optional = true }
diff --git a/components/metrics/lib.rs b/components/metrics/lib.rs
index 0ac3021c52b..8c19b129209 100644
--- a/components/metrics/lib.rs
+++ b/components/metrics/lib.rs
@@ -322,17 +322,32 @@ impl PaintTimeMetrics {
 
         if let Some(pending_metric) = self.pending_metrics.borrow_mut().remove(&epoch) {
             let profiler_metadata = pending_metric.0;
-            set_metric(
-                self,
-                profiler_metadata.clone(),
-                ProgressiveWebMetricType::FirstPaint,
-                ProfilerCategory::TimeToFirstPaint,
-                &self.first_paint,
-                paint_time,
-                &self.url,
-            );
-
-            if pending_metric.1 {
+
+            if self.first_paint.get().is_none() {
+                // TODO(delan): exclude compositor-to-script IPC time. May require support for
+                // backdating events, unless we make the compositor aware of things like the page
+                // URL for each pipeline and whether the paint was contentful.
+                #[cfg(feature = "tracing")]
+                tracing::info!(name: "First Paint", servo_profiling = true, url = self.url.to_string());
+
+                set_metric(
+                    self,
+                    profiler_metadata.clone(),
+                    ProgressiveWebMetricType::FirstPaint,
+                    ProfilerCategory::TimeToFirstPaint,
+                    &self.first_paint,
+                    paint_time,
+                    &self.url,
+                );
+            }
+
+            if self.first_contentful_paint.get().is_none() && pending_metric.1 {
+                // TODO(delan): exclude compositor-to-script IPC time. May require support for
+                // backdating events, unless we make the compositor aware of things like the page
+                // URL for each pipeline and whether the paint was contentful.
+                #[cfg(feature = "tracing")]
+                tracing::info!(name: "First Contentful Paint", servo_profiling = true, url = self.url.to_string());
+
                 set_metric(
                     self,
                     profiler_metadata,