protobuf: Add native histogram support

This is still experimental. It is not clear yet how to represent
native histograms in the text format, but with this commit,
implementers of OpenMetrics using the protobuf format can already
gather experience with native histograms.

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

proto/openmetrics_data_model.proto

@@ -158,6 +158,62 @@ message HistogramValue {
     // Optional.
     Exemplar exemplar = 3;
   }
+
+  // Everything below here is for native histograms (also known as sparse histograms).
+
+  // schema defines the bucket schema. Currently, valid numbers are -4 <= n <= 8.
+  // They are all for base-2 bucket schemas, where 1 is a bucket boundary in each case, and
+  // then each power of two is divided into 2^n logarithmic buckets.
+  // Or in other words, each bucket boundary is the previous boundary times 2^(2^-n).
+  // In the future, more bucket schemas may be added using numbers < -4 or > 8.
+  // Optional.
+  sint32 schema = 7;
+
+  // Breadth of the zero bucket.
+  // Optional.
+  double zero_threshold = 8;
+
+  // Count in zero bucket.
+  // Optional.
+  oneof zero_count {
+    double double_zero_count = 9;
+    uint64 int_zero_count = 10;
+  }
+
+  // Negative buckets for the native histogram.
+  // Optional.
+  repeated BucketSpan negative_span = 11;
+  // Use either "negative_delta" or "negative_count", the former for
+  // regular histograms with integer counts, the latter for float
+  // histograms.
+  repeated sint64 negative_delta    = 12; // Count delta of each bucket compared to previous one (or to zero for 1st bucket).
+  repeated double negative_count    = 13; // Absolute count of each bucket.
+
+  // Positive buckets for the native histogram.
+  // Optional.
+  repeated BucketSpan positive_span = 14;
+  // Use either "positive_delta" or "positive_count", the former for
+  // regular histograms with integer counts, the latter for float
+  // histograms.
+  repeated sint64 positive_delta    = 15; // Count delta of each bucket compared to previous one (or to zero for 1st bucket).
+  repeated double positive_count    = 16; // Absolute count of each bucket.
+
+  // A BucketSpan defines a number of consecutive buckets in a native
+  // histogram with their 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 BucketSpan {
+
+    // Gap to previous span, or starting point for 1st span (which can be negative).
+    // Required.
+    sint32 offset = 1;
+
+    // Length of consecutive buckets.
+    // Required.
+    uint32 length = 2;
+  }
 }
 
 message Exemplar {