Add log to capture the duplicate label if it exists in a metric

As suggested in PR

Signed-off-by: Gidesh Pampingal <gidesh.pampingal@johnlewis.co.uk>

collectors/monitoring_collector.go

@@ -389,7 +389,7 @@ func (c *MonitoringCollector) reportTimeSeriesMetrics(
 		// Add the metric labels
 		// @see https://cloud.google.com/monitoring/api/metrics
 		for key, value := range timeSeries.Metric.Labels {
-			if !keyExists(labelKeys, key) {
+			if !c.keyExists(labelKeys, key) {
 				labelKeys = append(labelKeys, key)
 				labelValues = append(labelValues, value)
 			}
@@ -398,7 +398,7 @@ func (c *MonitoringCollector) reportTimeSeriesMetrics(
 		// Add the monitored resource labels
 		// @see https://cloud.google.com/monitoring/api/resources
 		for key, value := range timeSeries.Resource.Labels {
-			if !keyExists(labelKeys, key) {
+			if !c.keyExists(labelKeys, key) {
 				labelKeys = append(labelKeys, key)
 				labelValues = append(labelValues, value)
 			}
@@ -511,9 +511,10 @@ func (c *MonitoringCollector) generateHistogramBuckets(
 	return buckets, nil
 }
 
-func keyExists(labelKeys []string, key string) bool {
+func (c *MonitoringCollector) keyExists(labelKeys []string, key string) bool {
 	for _, item := range labelKeys {
 		if item == key {
+			level.Debug(c.logger).Log("msg", "Found duplicate label key", "key", key)
 			return true
 		}
 	}