Real-user measurements: the ten most frequent script errors on /metrics too (live_rum_errors{message}), readable on the host without a staff session (#218)

ddiff --git a/server/index.js b/server/index.js
iindex e4eb181f..08aaf661 100644
---- a/server/index.js
++++ b/server/index.js
@@@ -880,6 +880,7 @@ observability.registerDomainGauges(metricsRegistry, {
      outboxStatus: async () => await require('./events/stream-events').counts(),
  });
  metricsRegistry.gauge({ name: 'live_rum_p75', help: 'Real-user p75 per page kind and device class (ms; cls unitless), from the newest beacons', labelNames: ['metric', 'page', 'device'], collect: () => rum.p75Gauge() });
++metricsRegistry.gauge({ name: 'live_rum_errors', help: 'Script errors visitors reported since this process started: the ten most frequent messages', labelNames: ['message'], collect: () => rum.errorsGauge() });
  app.get('/api/admin/rum', requireAuth, permissions.requireAdmin, (req, res) => { res.set('Cache-Control', 'no-store'); res.json(rum.snapshot()); });
  
  // ── Updates / Changelog ──────────────────────────────────────
ddiff --git a/server/web/rum.js b/server/web/rum.js
iindex a3b97e11..da3ff30f 100644
---- a/server/web/rum.js
++++ b/server/web/rum.js
@@@ -11,6 +11,8 @@
   *   POST /api/rum             the beacon (text/plain JSON from navigator.sendBeacon); always 204
   *   rum.snapshot()            p50/p75/p95 per page kind × device, and the most frequent errors (GET /api/admin/rum)
   *   rum.p75Gauge()            the p75s for /metrics (live_rum_p75{metric,page,device})
++ *   rum.errorsGauge()         the ten most frequent error messages for /metrics (live_rum_errors{message}), so they can
++ *                             be read on the host without a staff session
   */
  const express = require('express');
  
@@@ -113,7 +115,11 @@ function createRum({ now = () => Date.now() } = {}) {
          return out;
      }
  
--    return { router, record, snapshot, p75Gauge };
++    function errorsGauge() {
++        return [...errors.entries()].sort((a, b) => b[1] - a[1]).slice(0, 10).map(([message, count]) => ({ labels: { message: message.slice(0, 100) }, value: count }));
++    }
++
++    return { router, record, snapshot, p75Gauge, errorsGauge };
  }
  
  module.exports = { createRum, pageKind, deviceClass };
ddiff --git a/test/rum.test.js b/test/rum.test.js
iindex 1c541079..58328b3a 100644
---- a/test/rum.test.js
++++ b/test/rum.test.js
@@@ -56,6 +56,7 @@ const { createRum, pageKind, deviceClass } = require('../server/web/rum');
      const g = rum.p75Gauge();
      assert.ok(g.some((x) => x.labels.metric === 'lcp' && x.labels.page === 'home' && x.labels.device === 'phone' && x.value === 8000));
      assert.ok(!g.some((x) => x.labels.page === 'channel'), 'one channel sample is not a p75');
++    assert.deepStrictEqual(rum.errorsGauge().map((x) => [x.labels.message, x.value]), [['TypeError: x is undefined', 1], ['b', 1], ['c', 1]]);
  
      // A flood from one address: 20 a minute, then dropped until the next minute.
      const before = rum.snapshot().pages['clip|phone'];