Update bigtable latency histogram buckets to reflect reality. (#588)

* Update bigtable latency histogram buckets to reflect reality.

* Update comment to correct number of buckets

8 -> 6

Author: tomwilkie

pkg/chunk/gcp/instrumentation.go

@@ -20,9 +20,9 @@ var bigtableRequestDuration = prometheus.NewHistogramVec(prometheus.HistogramOpt
 	Name:      "bigtable_request_duration_seconds",
 	Help:      "Time spent doing Bigtable requests.",
 
-	// Bigtable latency seems to range from a few ms to a few sec and is
-	// important.  So use 8 buckets from 128us to 2s.
-	Buckets: prometheus.ExponentialBuckets(0.000128, 4, 8),
+	// Bigtable latency seems to range from a few ms to a few hundred ms and is
+	// important.  So use 6 buckets from 1ms to 1s.
+	Buckets: prometheus.ExponentialBuckets(0.001, 4, 6),
 }, []string{"operation", "status_code"})
 
 func init() {