Explain Span layout better

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

io/prometheus/client/metrics.proto

@@ -90,6 +90,12 @@ message Bucket {
 }
 
 message SparseBuckets {
+  // A Span is a given number of consecutive buckets at a given
+  // offset. Logically, it would be more straightforward to include
+  // the bucket counts in the Span. However, the protobuf
+  // representation is more compact in the way the data is structured
+  // here (with all the buckets in a single array separate from the
+  // Spans).
   message Span {
     optional sint32 offset = 1; // Gap to previous span, or starting point for 1st span (which can be negative).
     optional uint32 length = 2; // Length of consecutive buckets.