[metrics] No scrape-output integration test — exporter Prometheus format compliance unverified

[obchain]

Refs #50

File: crates/charon-metrics/tests/ (new)

Problem

Existing tests verify exporter binds an HTTP listener and that typed helpers don't panic. No test:

1. Records a counter/histogram observation
2. Scrapes /metrics
3. Asserts response contains expected # HELP, # TYPE, and metric lines per Prometheus text format

Without this, breakages in metrics crate version upgrade, label key changes, or histogram bucket ordering will only surface in production.

Fix

Add integration test in crates/charon-metrics/tests/scrape.rs:

#[tokio::test]
async fn scrape_returns_valid_prometheus_text() {
    let cfg = MetricsConfig { enabled: true, bind: "127.0.0.1:0".to_string(), .. };
    init(&cfg).await.unwrap();
    record_block_observed("bnb");
    let body = reqwest::get("http://127.0.0.1:<port>/metrics").await.unwrap().text().await.unwrap();
    assert!(body.contains("# HELP charon_scanner_blocks_total"));
    assert!(body.contains("charon_scanner_blocks_total{chain=\"bnb\"} 1"));
}

[obchain]

Resolved on feat/22-prometheus-metrics. See commits bd37bda (#221/#222/#223) and 7f6dc2f (#224).