metrics test: let the event-loop histogram take a sample before the scrape (the p99 line is correctly absent with no samples; CI raced it)

ddiff --git a/test/metrics.test.js b/test/metrics.test.js
iindex 655d6c7..1dbe0b8 100644
---- a/test/metrics.test.js
++++ b/test/metrics.test.js
@@@ -98,6 +98,9 @@ const metrics = require('../metrics');
          await get('/boom');
          await get('/sse');
          await get('/nope/9999/secret-path');
++        // The event-loop histogram only reports once it has samples; on a fast CI runner the scrape
++        // could otherwise come before the first sampling interval has elapsed.
++        await new Promise((r) => setTimeout(r, 150));
          const out = await get('/metrics');
          assert.strictEqual(out.status, 200);
          assert.match(out.type, /^text\/plain; version=0\.0\.4/);