fix: TRAC-1066 Reset stale queue gauges instead of holding last value

Cloudflare's queueBacklogAdaptiveGroups GraphQL analytics endpoint only
returns a row for a queue when there was measured activity in that
scrape window. When a queue goes idle, it silently disappears from the
response, and fetchQueueAnalytics only calls .Set() for queue IDs
present in the response — so the gauge freezes at its last non-zero
value forever instead of resetting.

This caused MakeswiftStencilWorkerQueueBacklogCritical/Warning to fire
in production reporting a 17.44k message backlog while the real queue
was confirmed empty via wrangler. Thanos showed the metric flatlined
at 691 for 13+ hours, then jumped to exactly 17444 and stayed frozen
there on both exporter replicas — the signature of a stale gauge, not
real queue depth.

Drop each account's previous label combinations for all 7 queue gauges
before repopulating them each scrape, mirroring the existing pattern
in fetchWorkerWAEAnalytics. Idle or deleted queues now disappear from
/metrics instead of holding stale values.

Fixes TRAC-1066
Co-Authored-By: Claude <noreply@anthropic.com>

Author: chanceaclark

prometheus.go

@@ -805,6 +805,18 @@ func fetchQueueAnalytics(account cfaccounts.Account, wg *sync.WaitGroup, deniedM
 
 	accountName := strings.ToLower(strings.ReplaceAll(account.Name, " ", "-"))
 
+	// Drop this account's previous label combinations so queues that have gone
+	// idle (or no longer exist) disappear instead of holding their last-seen
+	// value forever — Cloudflare's queueBacklogAdaptiveGroups only emits rows
+	// for queues with activity in the scrape window.
+	queueBacklogMessages.DeletePartialMatch(prometheus.Labels{"account": accountName})
+	queueBacklogBytes.DeletePartialMatch(prometheus.Labels{"account": accountName})
+	queueConsumerConcurrency.DeletePartialMatch(prometheus.Labels{"account": accountName})
+	queueOperations.DeletePartialMatch(prometheus.Labels{"account": accountName})
+	queueOperationBytes.DeletePartialMatch(prometheus.Labels{"account": accountName})
+	queueOperationLagTime.DeletePartialMatch(prometheus.Labels{"account": accountName})
+	queueOperationRetryCount.DeletePartialMatch(prometheus.Labels{"account": accountName})
+
 	resolveQueueName := func(queueID string) string {
 		if name, ok := names[queueID]; ok {
 			return name