Experimental encoding for sparse buckets in histogram

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

Author: beorn7

go/metrics.pb.go

@@ -60,6 +60,12 @@ message Histogram {
   optional uint64 sample_count = 1;
   optional double sample_sum   = 2;
   repeated Bucket bucket       = 3; // Ordered in increasing order of upper_bound, +Inf bucket is optional.
+  // Sparse bucket (sb) stuff:
+  optional uint32 sb_resolution      = 4; // That many buckets per power of ten. Must be <256. Zero for no sparse buckets.
+  optional double sb_zero_threshold  = 5; // Breadth of the zero bucket.
+  optional uint64 sb_zero_count      = 6; // Count in zero bucket.
+  optional SparseBuckets sb_negative = 7; // Negative sparse buckets.
+  optional SparseBuckets sb_positive = 8; // Positive sparse buckets.
 }
 
 message Bucket {
@@ -68,6 +74,15 @@ message Bucket {
   optional Exemplar exemplar         = 3;
 }
 
+message SparseBuckets {
+  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.
+  }
+  repeated Span span    = 1;
+  repeated sint64 delta = 2; // Count delta of each bucket compared to previous one (or to zero for 1st bucket).
+}
+
 message Exemplar {
   repeated LabelPair                 label     = 1;
   optional double                    value     = 2;