docs(contrib): Document chrome tracing, rather than bespoke profiling

Author: epage

src/cargo/util/profile.rs

@@ -2,13 +2,20 @@
 
 ## Internal profiler
 
-Cargo has a basic, hierarchical profiler built-in. The environment variable
-`CARGO_PROFILE` can be set to an integer which specifies how deep in the
-profile stack to print results for.
+Cargo leverages [tracing](https://crates.io/crates/tracing)
+as a basic, hierarchical built-in profiler.
 
-```sh
-# Output first three levels of profiling info
-CARGO_PROFILE=3 cargo generate-lockfile
+Environment variables:
+- `__CARGO_LOG_PROFILE=<true|1>`: log tracing events to a file in the current working directory
+- `__CARGO_LOG_PROFILE_CAPTURE_ARGS=<true|1>`: include arguments in the events
+
+At process exit, your trace will be in a file like `trace-1668480819035032.json`.
+Open that file with [ui.perfetto.dev](https://ui.perfetto.dev) (or chrome://tracing) to browse your trace.
+
+Example:
+```console
+$ # Output first three levels of profiling info
+$ __CARGO_LOG_PROFILE=true cargo generate-lockfile
 ```
 
 ## Benchmarking