test: isolate Stream metrics rendering

[smiggleworth]

Summary

• let the Stream Prometheus renderer consume an explicit metrics collector internally
• exercise exact cached gauge rendering with a test-local collector
• prevent unrelated parallel Stream tests from overwriting values under assertion

Why

The main-branch run after #233 failed because should_render_stream_metrics_from_cached_observability_state wrote exact values to the process-global collector while unrelated tests updated the same gauges concurrently. PR CI had passed the identical product commit; this removes the test data race instead of rerunning it.

Failure evidence: https://github.com/cntryl/fitz/actions/runs/33771460403/job/100702453142

Validation

• cargo fmt --all -- --check
• cargo clippy --locked --workspace --all-targets --all-features -- -D warnings -D clippy::pedantic
• cargo test --locked --workspace
• focused Stream cached-observability renderer test