Implement Summary point value type in Otel/STEF (#116)

Resolves #5

Implements support for Otel's Summary data point type:
https://github.com/open-telemetry/opentelemetry-proto/blob/6deb067376dc017240f940c252b8b116fd3becf1/opentelemetry/proto/metrics/v1/metrics.proto#L208

Author: tigrannajaryan

benchmarks/testdata/astronomy-otelmetrics.stefz

@@ -57,7 +57,7 @@ oneof PointValue {
   Float64 float64
   Histogram HistogramValue
   ExpHistogram ExpHistogramValue
-  // TODO: Add Summary value support.
+  Summary SummaryValue
 }
 
 struct Metric dict(Metric) {
@@ -130,6 +130,17 @@ struct ExpHistogramBuckets {
   BucketCounts []uint64
 }
 
+struct SummaryValue {
+  Count uint64
+  Sum float64
+  QuantileValues []QuantileValue
+}
+
+struct QuantileValue {
+  Quantile float64
+  Value float64
+}
+
 oneof AnyValue {
   String string dict(AnyValueString)
   Bool bool