Set RUST_BACKTRACE=1 for production services: create a crate and use it

[sunshowers]

While debugging an instance of #2416, I saw at gc08's /pool/ext/8a199f12-4f5c-483a-8aca-f97856658a35/crypt/debug/oxz_nexus_65a11c18-7f59-41ac-b9e7-680627f996e7/oxide-nexus:default.log.1711665000:

thread 'tokio-runtime-worker' panicked at nexus/db-queries/src/db/sec_store.rs:65:60:
called `Result::unwrap()` on an `Err` value: InternalError { internal_message: "database error (kind = Unknown): result is ambiguous: error=rpc error: code = Unavailable desc = error reading from server: read tcp [fd00:1122:3344:109::3]:56722->[fd00:1122:3344:105::3]:32221: read: connection reset by peer [exhausted]\n" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
[ Mar 28 22:01:58 Stopping because all processes in service exited. ]
[ Mar 28 22:01:58 Executing stop method (:kill). ]

In this case the issue is pretty clear, but I'm wondering if we've considered setting RUST_BACKTRACE=1 in our production environment. Having backtraces is something that can definitely aid in debugging, but maybe it isn't a big deal because the core file can show what's going on. (But see #5359.)

According to https://stackoverflow.com/questions/29421727/how-much-overhead-does-rust-backtrace-1-have it seems like there's some performance cost, so we'd have to measure it carefully.

Wonder if @hawkw has thoughts here.

### Tasks
- [ ] Create a small crate to set RUST_BACKTRACE=1 if it isn't set already (and maybe RUST_LIB_BACKTRACE as well)
- [ ] Use the crate in nexus
- [ ] Use the crate in sled-agent
- [ ] Use it in wicketd
- [ ] Use it elsewhere (add tasks for other services that would benefit)

[davepacheco]

I definitely think this is worthwhile. Even in environments where I've had easy access to core files and easy ways to get stuff like a stack trace out, it was still very valuable to have the runtime-printed stack trace available.

[jclulow]

Is it possible to build the binary so it defaults to this behaviour, instead of requiring the environment be set?

[hawkw]

Wonder if @hawkw has thoughts here.

I don't have a strong opinion --- IMO, we should have some way of collecting backtraces from production crashes; if we can get this data from the core, it's maybe less pressing, but I would err on the side of including them.

[jgallagher]

Is it possible to build the binary so it defaults to this behaviour, instead of requiring the environment be set?

It doesn't look like it. The default panic hook calls get_backtrace_style, which reads from RUST_BACKTRACE unless set_backtrace_style has been called. set_backtrace_style is unstable (rust-lang/rust#93346), so we can't call that even if we wanted to.

We could set RUST_BACKTRACE from inside the program, presumably as one of the first thing we do in main? 😬 Gross, but it works, so I thought I'd mention it.

[sunshowers]

I like the idea of setting RUST_BACKTRACE=1 within main. I'd do something like if RUST_BACKTRACE isn't set, set it to 1.

[sunshowers]

(Note to people in the future wondering -- setting a panic hook isn't enough. For example, anyhow reads RUST_BACKTRACE).

[sunshowers]

Another consideration here is whether we want anyhow to also capture backtraces. That is controlled with both RUST_BACKTRACE and another env var, RUST_LIB_BACKTRACE. anyhow documents the logic.

[sunshowers]

rust-analyzer uses the same strategy too, so we'd be in good company:

https://github.com/rust-lang/rust-analyzer/blob/c3b8c2a25413e2aa58295d18c12902a624471b74/crates/rust-analyzer/src/bin/main.rs#L105-L107