Avoid the term 'sparse' where possible

This intends to avoid confusing users by the subtle difference between
a native histogram and a sparse bucket.

Signed-off-by: beorn7 <beorn@grafana.com>

Author: beorn7

examples/random/main.go

@@ -68,7 +68,7 @@ func main() {
 			Name:                "rpc_durations_histogram_seconds",
 			Help:                "RPC latency distributions.",
 			Buckets:             prometheus.LinearBuckets(*normMean-5**normDomain, .5**normDomain, 20),
-			SparseBucketsFactor: 1.1,
+			NativeHistogramBucketFactor: 1.1,
 		})
 	)