Add in opentelemetry tracing as a feature

This is an evolution on the previous tracing support I added a few years
back. See CONTRIBUTING for details.

The rs_tracing crate is functionally fine but very limited in features
compared to the tracing crate that provides the developer interface for
opentelemetry instrumentation. As we start looking at performance again
I think this tooling will be useful.

Author: rbtcollins

CONTRIBUTING.md

@@ -217,8 +217,8 @@ break our testing (like `RUSTUP_TOOLCHAIN`, `SHELL`, `ZDOTDIR`, `RUST_BACKTRACE`
 But if you want to debug locally, you may need backtrace. `RUSTUP_BACKTRACE`
 is used like `RUST_BACKTRACE` to enable backtraces of failed tests.
 
-**NOTE**: This is a backtrace for the test, not for any rustup process running
-in the test
+**NOTE**: This is a backtrace for the test, not for any subprocess invocation of
+rustup process running in the test
 
 ```bash
 $ RUSTUP_BACKTRACE=1 cargo test --release --test cli-v1 -- remove_toolchain_then_add_again
@@ -273,3 +273,62 @@ test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 26 filtered out
 
 error: test failed, to rerun pass '--test cli-v1'
 ```
+
+## Tracing
+
+The feature "otel" can be used when building rustup to turn on Opentelemetry
+tracing with a Jaeger exporter.
+
+The normal Jaeger environment variables (`ENV_AGENT_HOST`, `ENV_AGENT_PORT`),
+can be used to customise its behaviour, but often the simplest thing is to just
+run a Jaeger docker container on the same host:
+
+```sh
+docker run -d --name jaeger   -e COLLECTOR_ZIPKIN_HOST_PORT=:9411   -e COLLECTOR_OTLP_ENABLED=true   -p 6831:6831/udp   -p 6832:6832/udp   -p 5778:5778   -p 16686:16686   -p 4317:4317   -p 4318:4318   -p 14250:14250   -p 14268:14268   -p 14269:14269   -p 9411:9411   jaegertracing/all-in-one:latest
+```
+
+Then build rustup-init with tracing:
+
+```sh
+cargo build --features=otel
+```
+
+Run the operation you want to analyze:
+
+```sh
+RUSTUP_FORCE_ARG0="rustup" ./target/debug/rustup-init.exe show
+```
+
+And [look in Jaeger for a trace](http://localhost:16686/search?service=rustup).
+
+### Adding instrumentation
+
+The `otel` feature uses conditional compilation to only add function instrument
+when enabled. Instrumenting a currently uninstrumented function is mostly simply
+done like so:
+
+```rust
+#[cfg_attr(feature = "otel", tracing::instrument(skip_all))]
+```
+
+`skip_all` is not required, but some core structs don't implement Debug yet, and
+others have a lot of output in Debug : tracing adds some overheads, so keeping
+spans lightweight can help avoid frequency bias in the results - where
+parameters with large debug in frequently called functions show up as much
+slower than they are.
+
+Some good general heuristics:
+
+- Do instrument slow blocking functions
+- Do instrument functions with many callers or that call many different things,
+  as these tend to help figure the puzzle of what-is-happening
+- Default to not instrumenting thin shim functions (or at least, only instrument
+  them temporarily while figuring out the shape of a problem)
+
+### Caveats
+
+Cross-thread propogation isn't connected yet. This will cause instrumentation in
+a thread to make a new root span until it is fixed. If any Tokio runtime-related
+code gets added in those threads this will also cause a panic. We have a couple
+of threadpools in use today; if you need to instrument within that context, use
+a thunk to propogate the tokio runtime into those threads.