Clarify new bucket store metrics (#3229)

* Clarify new bucket store metrics

Signed-off-by: Marco Pracucci <marco@pracucci.com>

* Improved metrics help message based on feedback received

Signed-off-by: Marco Pracucci <marco@pracucci.com>

pkg/store/bucket.go

@@ -226,13 +226,13 @@ func newBucketStoreMetrics(reg prometheus.Registerer) *bucketStoreMetrics {
 
 	m.seriesFetchDuration = promauto.With(reg).NewHistogram(prometheus.HistogramOpts{
 		Name:    "thanos_bucket_store_cached_series_fetch_duration_seconds",
-		Help:    "Time it takes to fetch series from a bucket to respond a query. It also includes the time it takes to cache fetch and store operations.",
+		Help:    "The time it takes to fetch series to respond to a request sent to a store gateway. It includes both the time to fetch it from the cache and from storage in case of cache misses.",
 		Buckets: []float64{0.001, 0.01, 0.1, 0.3, 0.6, 1, 3, 6, 9, 20, 30, 60, 90, 120},
 	})
 
 	m.postingsFetchDuration = promauto.With(reg).NewHistogram(prometheus.HistogramOpts{
 		Name:    "thanos_bucket_store_cached_postings_fetch_duration_seconds",
-		Help:    "Time it takes to fetch postings from a bucket to respond a query. It also includes the time it takes to cache fetch and store operations.",
+		Help:    "The time it takes to fetch postings to respond to a request sent to a store gateway. It includes both the time to fetch it from the cache and from storage in case of cache misses.",
 		Buckets: []float64{0.001, 0.01, 0.1, 0.3, 0.6, 1, 3, 6, 9, 20, 30, 60, 90, 120},
 	})