refactor(stats): take stats_url as a StatsFlusher::new parameter

[lucaspimentel]

Summary of changes

Refactor StatsFlusher::new to accept a stats_url: String parameter instead of deriving the URL internally from config.site.

Reason for change

1. Removes internal duplication — trace_stats_url(&self.config.site) was being called in two separate places inside StatsFlusher (in get_or_init and again in the retry loop). It's now computed once at the call site.
2. Enables future flexibility — callers can supply any URL (e.g. a custom endpoint or test harness) without StatsFlusher needing to know about it.
3. Better separation of concerns — StatsFlusher no longer owns URL resolution; it simply uses the URL it's given. This mirrors how TraceFlusher already works, where the URL comes from outside via SendData's embedded Endpoint.

Implementation details

• Added a stats_url: String field to StatsFlusher and a corresponding parameter to StatsFlusher::new.
• Removed the trace_stats_url import from stats_flusher.rs; the call now lives in main.rs::start_trace_agent.
• Replaced the two internal trace_stats_url(&self.config.site) calls with self.stats_url.
• Retry-loop debug logs now report endpoint.url so they reflect the URL actually contacted instead of a re-derived site URL.

Test coverage

No new tests — this is a pure refactor of an internal constructor signature with no behavior change. Verified existing checks pass:

• cargo build on the bottlecap crate
• cargo check --workspace — clean
• RUSTFLAGS="-D warnings" cargo clippy --workspace --all-targets --features default — clean
• cargo fmt --all -- --check — clean
• cargo nextest run --workspace — 526/526 passed

[Copilot]

Pull request overview

Refactors StatsFlusher so its stats intake URL is provided by the caller instead of being derived internally, enabling future use cases like redirecting stats flushing to alternate endpoints (e.g., a fake intake for integration tests).

Changes:

• Add a stats_url: String parameter to StatsFlusher::new and store it on the flusher.
• Move trace_stats_url(&config.site) computation into bottlecap’s main.rs at the call site.
• Update retry-loop debug logs to report the actual endpoint.url used for requests.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
bottlecap/src/traces/stats_flusher.rs	Accepts/stores stats_url, uses it to build the Endpoint, and logs the contacted endpoint URL.
bottlecap/src/bin/bottlecap/main.rs	Computes trace_stats_url(&config.site) and passes it into StatsFlusher::new.