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

[lucaspimentel]

Overview

StatsFlusher derived its intake URL internally via trace_stats_url(&config.site), which hardcodes https://trace.agent.{site}/api/v0.2/stats with no override hook. This PR lifts the URL to the caller: StatsFlusher::new now takes a stats_url: String parameter, and main.rs computes trace_stats_url(&config.site) at the call site.

This mirrors how TraceFlusher already works (URL comes from outside via SendData's embedded Endpoint) and unblocks redirecting StatsFlusher to alternate endpoints, which is needed for the upcoming in-process fake-intake test harness (#1194).

Retry-loop debug logs now report endpoint.url so they reflect the URL actually contacted instead of a re-derived site URL.

This refactor was split out of #1194 per review feedback to keep the production signature change separate from the test-harness addition.

Testing

• 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
• CI green

[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.