histogram: Add a doc comment about when to add a no-op span

See prometheus/client_golang#1127 for details.

Signed-off-by: beorn7 <[email protected]>

io/prometheus/client/metrics.proto

@@ -98,6 +98,9 @@ message Histogram {
   repeated double negative_count    = 11; // Absolute count of each bucket.
 
   // Positive buckets for the native histogram.
+  // Use a no-op span (offset 0, length 0) for a native histogram without any
+  // observations yet and with a zero_threshold of 0. Otherwise, it would be
+  // indistinguishable from a classic histogram.
   repeated BucketSpan positive_span = 12;
   // Use either "positive_delta" or "positive_count", the former for
   // regular histograms with integer counts, the latter for float