Auto merge of #143525 - Shourya742:2025-07-06-add-profiler, r=Kobzol

Add profiler to bootstrap command

This PR adds command profiling to the bootstrap command. It tracks the total execution time and records cache hits for each command. It also provides the ability to export execution result to a JSON file. Integrating this with Chrome tracing could further enhance observability.

r? `@Kobzol`

Author: bors

src/building/bootstrapping/debugging-bootstrap.md

@@ -168,10 +168,17 @@ For `#[instrument]`, it's recommended to:
 
 ### Profiling bootstrap
 
-You can use the `COMMAND` tracing target to trace execution of most commands spawned by bootstrap. If you also use the `BOOTSTRAP_PROFILE=1` environment variable, bootstrap will generate a Chrome JSON trace file, which can be visualized in Chrome's `chrome://tracing` page or on https://ui.perfetto.dev.
+You can set the `BOOTSTRAP_PROFILE=1` environment variable to enable command execution profiling during bootstrap. This generates:
+
+* A Chrome trace file (for visualization in `chrome://tracing` or [Perfetto](https://ui.perfetto.dev)) if tracing is enabled via `BOOTSTRAP_TRACING=COMMAND=trace`
+* A plain-text summary file, `bootstrap-profile-{pid}.txt`, listing all commands sorted by execution time (slowest first), along with cache hits and working directories
+
+Note: the `.txt` report is always generated when `BOOTSTRAP_PROFILE=1` is set — tracing is not required.
+
+Example usage:
 
 ```bash
-$ BOOTSTRAP_TRACING=COMMAND=trace BOOTSTRAP_PROFILE=1 ./x build library
+$ BOOTSTRAP_PROFILE=1 BOOTSTRAP_TRACING=COMMAND=trace ./x build library
 ```
 
 ### rust-analyzer integration?