feat(http): expose upstream_states and request timing/byte accessors

[u5surf]

Proposed changes

Add safe accessors for request struct fields that previously required raw-pointer deref:

• Request::upstream_states() -> &[UpstreamState] walks each upstream attempt (one per peer for proxy_next_upstream retries) via a #[repr(transparent)] wrapper exposing peer, status, response_time, connect_time, header_time, queue_time, bytes_sent, bytes_received, response_length.
• Request::start_sec(), start_msec(), request_length(), bytes_sent() for the request-level timing/byte fields.

Request::upstream() (the raw-pointer accessor) is left untouched to preserve compatibility.

Unit tests follow the MaybeUninit::zeroed pattern from #272.

Checklist

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• I have written my commit messages in the Conventional Commits format.
• I have read the CONTRIBUTING doc
• I have added tests (when possible) that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[avahahn]

Just a few minor nits. Otherwise this MR does what it says it does in a relatively safe way.

[avahahn]

We may prefer in the future to have a separate type for surfacing ngx_connection_t details to the caller so It may be best to not litter request with details of it for now, but I will defer to @bavshin-f5 on that.

[avahahn]

this final assert statement is simply testing ngx_str_t.

[avahahn]

I wonder if we can use a lifetime on the returned slice to inform rust that the lifetime is equivalent to the lifetime of the self reference.

[avahahn]

again here: I wonder if we can set the lifetime of the NgxStr to an identical lifetime attached to self.

[u5surf]

Thanks for the review, and sorry for the delay. Rebased onto main and addressed the nits.

upstream.rs test assert — dropped; the test now just checks that peer() resolves a populated ngx_str_t, without re-asserting what NgxStr already covers.

Lifetimes on the returned slice and NgxStr — these are already tied to &self by elision. fn upstream_states(&self) -> &[UpstreamState] desugars to fn upstream_states<'a>(&'a self) -> &'a [UpstreamState], and the same holds for UpstreamState::peer. Writing them out explicitly makes clippy ask for them back:

warning: the following explicit lifetimes could be elided: 'a
   --> src/http/request.rs:236:28
    = note: `#[warn(clippy::needless_lifetimes)]` on by default

which would fail CI under RUSTFLAGS: --deny warnings. So I have left them elided. Happy to add a doc note spelling the borrow out instead, if that would read better.

ngx_connection_t details on Request — leaving as is pending @bavshin-f5, since you deferred that one. Easy to move behind a separate type later if that is the direction.

cargo fmt, clippy --all-targets and the test suite are clean on nginx 1.30.4.