Base-2 exponential histogram protocol support

CHANGELOG.md

@@ -16,7 +16,7 @@ Full list of differences found in [this compare.](https://github.com/open-teleme
 
 ### Added
 
-* Remove if no changes for this section before release.
+* ExponentialHistogram is a base-2 exponential histogram described in [OTEP 149](https://github.com/open-telemetry/oteps/pull/149).
 
 ### Removed
 

opentelemetry/proto/metrics/v1/metrics.proto

@@ -221,6 +221,16 @@ message Histogram {
   AggregationTemporality aggregation_temporality = 2;
 }
 
+// ExponentialHistogram represents the type of a metric that is calculated by aggregating
+// as a ExponentialHistogram of all reported double measurements over a time interval.
+message ExponentialHistogram {
+  repeated ExponentialHistogramDataPoint data_points = 1;
+
+  // aggregation_temporality describes if the aggregator reports delta changes
+  // since last report time, or cumulative changes since a fixed start time.
+  AggregationTemporality aggregation_temporality = 2;
+}
+
 // Summary metric data are used to convey quantile summaries,
 // a Prometheus (see: https://prometheus.io/docs/concepts/metric_types/#summary)
 // and OpenMetrics (see: https://github.com/OpenObservability/OpenMetrics/blob/4dbf6075567ab43296eed941037c12951faafb92/protos/prometheus.proto#L45)
@@ -431,6 +441,134 @@ message HistogramDataPoint {
   repeated Exemplar exemplars = 8;
 }
 
+// ExponentialHistogramDataPoint is a single data point in a timeseries that describes the
+// time-varying values of a ExponentialHistogram of double values. A ExponentialHistogram contains
+// summary statistics for a population of values, it may optionally contain the
+// distribution of those values across a set of buckets.
+//
+message ExponentialHistogramDataPoint {
+  // The set of key/value pairs that uniquely identify the timeseries from
+  // where this point belongs. The list may be empty (may contain 0 elements).
+  repeated opentelemetry.proto.common.v1.KeyValue attributes = 1;
+
+  // StartTimeUnixNano is optional but strongly encouraged, see the
+  // the detiled comments above Metric.
+  //
+  // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January
+  // 1970.
+  fixed64 start_time_unix_nano = 2;
+
+  // TimeUnixNano is required, see the detailed comments above Metric.
+  //
+  // Value is UNIX Epoch time in nanoseconds since 00:00:00 UTC on 1 January
+  // 1970.
+  fixed64 time_unix_nano = 3;
+
+  // count is the number of values in the population. Must be non-negative. This
+  // value must be equal to the sum of the "count" fields in buckets if a
+  // histogram is provided.
+  fixed64 count = 4;
+
+  // sum of the values in the population. If count is zero then this field
+  // must be zero. This value must be equal to the sum of the "sum" fields in
+  // buckets if a histogram is provided.
+  //
+  // Note: Sum should only be filled out when measuring non-negative discrete
+  // events, and is assumed to be monotonic over the values of these events.
+  // Negative events *can* be recorded, but sum should not be filled out when
+  // doing so.  This is specifically to enforce compatibility w/ OpenMetrics,
+  // see: https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#histogram
+  double sum = 5;
+
+  // scale describes the resolution of the histogram.  Boundaries are
+  // located at powers of the base, where:
+  //
+  //   base = (2^(2^-scale))
+  //
+  // The histogram bucket identified by `index`, a signed integer,
+  // contains values that are less than or equal to (base^index) and
+  // greater than (base^(index-1)).
+  //
+  // The positive and negative ranges of the histogram are expressed
+  // separately but both use the same scale to locate their boundaries.
+  //
+  // scale is valid in the range -4 through +8 inclusive.
+  sint32 scale = 6;
+
+  // zero_count is the count of values that are either exactly zero or
+  // within the region considered zero by the instrumentation at the
+  // tolerated degree of precision.  This bucket stores values that
+  // cannot be expressed using the standard exponential formula as
+  // well as values that have been rounded to zero.  Users have the
+  // option to set zero_tolerance to convey additional information about
+  // the width of the zero region.
+  //
+  // Implementations MAY consider the zero bucket to have probability
+  // mass equal to (zero_count / sum).
+  fixed64 zero_count = 7;
+
+  // zero_tolerance may be optionally set to convey the width of the
+  // zero region.  When this is not set, implementations may assume
+  // the zero tolerance equals the smallest representable positive
+  // IEEE 754 double-precision floating point number, which is (2^−1074).
+  //
+  // Producers can be configured to round measurements to zero that fall
+  // below the zero tolerance.  Users should avoid producing measurements
+  // that have meaningless precision near zero, as exponential buckets
+  // become increasingly dense in this region.
+  //
+  // Implementations MAY consider the zero bucket to have probability
+  // density equal to (zero_count / 2*zero_tolerance), if set.
+  double zero_tolerance = 8;
+
+  // positive carries the positive range of exponential bucket counts.
+  SparseBuckets positive = 9;
+
+  // negative carries the negative range of exponential bucket counts.
+  SparseBuckets negative = 10;
+
+  // SparseBuckets are a sparse set of bucket counts, compactly encoded.
+  message SparseBuckets {
+    // Span encodes a run of contiguous histogram buckets.
+    message Span {
+      sint32 offset = 1; // Gap to previous span, or starting point for 1st span (which can be negative).
+      uint32 length = 2; // Length of consecutive buckets.
+    }
+
+    // span is a repeated list of Spans that encodes a range of
+    // exponential buckets, positive or negative.  The offset of the
+    // first Span indicates the smallest index in the Histogram with
+    // non-zero count.  Subsequent contiguous buckets will be included
+    // in the same Span, and subsequent Spans encode additional
+    // buckets separated by gaps where the histogram has zero count.
+    //
+    // The starting index of a Span equals its own offset plus the
+    // starting index of the previous Span plus the previous Span's
+    // length.  As a recursive formula:
+    //
+    //    starting_index(span[x+1]) =
+    //       starting_index(span[x]) +
+    //       span[x+1].offset +
+    //       span[x].length
+    //
+    // For example, the two Spans [ {offset: 10, length: 3}, {offset:
+    // 5, length: 8} ] indicate a total of 11 buckets with indices at
+    // [10, 11, 12] and [18, 19, 20, 21, 22, 23, 24, 25].
+    repeated Span span = 1;
+
+    // delta is an array of counts corresponding to the buckets
+    // described by `span`, encoded as deltas.  The indicated count 
+    // for bucket[x] is defined as a recursive formula:
+    //
+    //    bucket_count(x+1) = bucket_count(x) + delta[x+1]
+    repeated sint64 delta = 2;
+  } 
+
+  // (Optional) List of exemplars collected from
+  // measurements that were used to form the data point
+  repeated Exemplar exemplars = 8;
+}
+
 // SummaryDataPoint is a single data point in a timeseries that describes the
 // time-varying values of a Summary metric.
 message SummaryDataPoint {