feat: update prometheus remote write proto file for native histogram (#294)

* feat: update prometheus remote write proto file for native histogram

* chore: format

proto/prometheus/remote/types.proto

@@ -31,7 +31,7 @@ message MetricMetadata {
   }
 
   // Represents the metric type, these match the set from Prometheus.
-  // Refer to model/textparse/interface.go for details.
+  // Refer to github.com/prometheus/common/model/metadata.go for details.
   MetricType type = 1;
   string metric_family_name = 2;
   string help = 4;
@@ -54,6 +54,75 @@ message Exemplar {
   int64 timestamp = 3;
 }
 
+// A native histogram, also known as a sparse histogram.
+// Original design doc:
+// https://docs.google.com/document/d/1cLNv3aufPZb3fNfaJgdaRBZsInZKKIHo9E6HinJVbpM/edit
+// The appendix of this design doc also explains the concept of float
+// histograms. This Histogram message can represent both, the usual
+// integer histogram as well as a float histogram.
+message Histogram {
+  enum ResetHint {
+    UNKNOWN = 0; // Need to test for a counter reset explicitly.
+    YES = 1; // This is the 1st histogram after a counter reset.
+    NO = 2; // There was no counter reset between this and the previous Histogram.
+    GAUGE = 3; // This is a gauge histogram where counter resets don't happen.
+  }
+
+  oneof count { // Count of observations in the histogram.
+    uint64 count_int = 1;
+    double count_float = 2;
+  }
+  double sum = 3; // Sum of observations in the histogram.
+  // The 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.
+  sint32 schema = 4;
+  double zero_threshold = 5; // Breadth of the zero bucket.
+  oneof zero_count { // Count in zero bucket.
+    uint64 zero_count_int = 6;
+    double zero_count_float = 7;
+  }
+
+  // Negative Buckets.
+  repeated BucketSpan negative_spans = 8;
+  // Use either "negative_deltas" or "negative_counts", the former for
+  // regular histograms with integer counts, the latter for float
+  // histograms.
+  repeated sint64 negative_deltas = 9; // Count delta of each bucket compared to previous one (or to zero for 1st bucket).
+  repeated double negative_counts = 10; // Absolute count of each bucket.
+
+  // Positive Buckets.
+  repeated BucketSpan positive_spans = 11;
+  // Use either "positive_deltas" or "positive_counts", the former for
+  // regular histograms with integer counts, the latter for float
+  // histograms.
+  repeated sint64 positive_deltas = 12; // Count delta of each bucket compared to previous one (or to zero for 1st bucket).
+  repeated double positive_counts = 13; // Absolute count of each bucket.
+
+  ResetHint reset_hint = 14;
+  // timestamp is in ms format, see model/timestamp/timestamp.go for
+  // conversion from time.Time to Prometheus timestamp.
+  int64 timestamp = 15;
+
+  // custom_values are not part of the specification, DO NOT use in remote write clients.
+  // Used only for converting from OpenTelemetry to Prometheus internally.
+  repeated double custom_values = 16;
+}
+
+// A BucketSpan defines a number of consecutive buckets 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 {
+  sint32 offset = 1; // Gap to previous span, or starting point for 1st span (which can be negative).
+  uint32 length = 2; // Length of consecutive buckets.
+}
+
 // TimeSeries represents samples and labels for a single time series.
 message TimeSeries {
   // For a timeseries to be valid, and for the samples and exemplars
@@ -61,6 +130,7 @@ message TimeSeries {
   repeated Label labels = 1;
   repeated Sample samples = 2;
   repeated Exemplar exemplars = 3;
+  repeated Histogram histograms = 4;
 }
 
 message Label {
@@ -105,6 +175,8 @@ message Chunk {
   enum Encoding {
     UNKNOWN = 0;
     XOR = 1;
+    HISTOGRAM = 2;
+    FLOAT_HISTOGRAM = 3;
   }
   Encoding type = 3;
   bytes data = 4;