Add module count field to module graph tracing spans (#91697)
### What?
Adds a `modules` field to two tracing spans that cover module graph
construction:
- `"module graph for endpoint"` span (per-page graph, in `app.rs`) —
records the total number of modules across all `SingleModuleGraph`s
built for that endpoint.
- `"whole app module graph"` span (new info-level span wrapping
`whole_app_module_graph_operation` in `project.rs`) — records the
combined module count of the base graph and the additional-entries
graph.
Also adds a `module_count()` method to `SingleModuleGraph` (as a
`#[turbo_tasks::function]` returning `Vc<u64>`) that exposes
`number_of_modules`.
### Why?
Module graph size directly affects build and HMR performance. Without a
count in the trace, it is impossible to correlate span duration with
graph size or spot regressions where the graph grows unexpectedly.
Surfacing the count as a structured span field makes it queryable
without requiring log parsing.
### How?
- `SingleModuleGraph::module_count()` is a cheap
`#[turbo_tasks::function]` that wraps the already-computed
`number_of_modules: usize` field as a `Vc<u64>`.
- Both call sites follow the deferred-field pattern already used in
`ProjectContainer::initialize`: the span is created with `modules =
Empty` before the `async move` block, `span.record("modules", count)` is
called once the graphs are ready, and `span_clone` is passed to
`.instrument()` so the field is recorded on the correct span.
- Module counts are summed across all graphs that contribute to the
endpoint/app.
---------
Co-authored-by: Tobias Koppers <sokra@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com>
