Implement Conversion Between OC and OTel Summary Metric (#2048)

_Note:_ This PR is based off the Summary datapoint proto (that has not been merged upstream) in this [PR](https:/open-telemetry/opentelemetry-proto/pull/227/files).

**Description:** 

This PR has two main parts: 

* Implement the [Summary datapoint proto](https:/open-telemetry/opentelemetry-proto/pull/227/files) in the OTel Collector. Changes were made to _metrics_struct.go_ to create the generated Summary structs (using the `make genproto` and `make genpdata` commands).
* Add in the conversion between OpenCensus summary metrics and OTel Summary datapoints. This conversion directly adds compatibility for the Prometheus Summary metric in the PrometheusReceiver. 

**Link to tracking Issue:** [opentelemetry-specification/issues/1146](open-telemetry/opentelemetry-specification#1146) and [opentelemetry-collector/issues/1638](#1638)

A lot of the code in this PR is auto-generated from creating the structs and protos. The following files are were auto-generated:
* generated_metrics.go
* generated_metrics_test.go
* trace_config.pb.go 
* metrics.pg.go

**Testing:**

* Updated unit tests in _oc_to_metrics.go_ for conversion between OC summary metrics to OTel summary metrics
* Created end-to-end tests for the Summary metric in PrometheusReceiver

cmd/pdatagen/internal/metrics_structs.go

@@ -39,6 +39,7 @@ var metricsFile = &File{
  doubleSum,
  intHistogram,
  doubleHistogram,
+ doubleSummary,
  intDataPointSlice,
  intDataPoint,
  doubleDataPointSlice,
@@ -47,6 +48,10 @@ var metricsFile = &File{
  intHistogramDataPoint,
  doubleHistogramDataPointSlice,
  doubleHistogramDataPoint,
+ doubleSummaryDataPointSlice,
+ doubleSummaryDataPoint,
+ quantileValuesSlice,
+ quantileValues,
  intExemplarSlice,
  intExemplar,
  doubleExemplarSlice,
@@ -206,6 +211,19 @@ var doubleHistogram = &messageStruct{
  },
 }
 
+var doubleSummary = &messageStruct{
+ structName: "DoubleSummary",
+ description: "// DoubleSummary represents the type of a metric that is calculated by aggregating as a Summary of all reported double measurements over a time interval.",
+ originFullName: "otlpmetrics.DoubleSummary",
+ fields: []baseField{
+ &sliceField{
+ fieldName: "DataPoints",
+ originFieldName: "DataPoints",
+ returnSlice: doubleSummaryDataPointSlice,
+ },
+ },
+}
+
 var intDataPointSlice = &sliceStruct{
  structName: "IntDataPointSlice",
  element: intDataPoint,
@@ -284,6 +302,44 @@ var doubleHistogramDataPoint = &messageStruct{
  },
 }
 
+var doubleSummaryDataPointSlice = &sliceStruct{
+ structName: "DoubleSummaryDataPointSlice",
+ element: doubleSummaryDataPoint,
+}
+
+var doubleSummaryDataPoint = &messageStruct{
+ structName: "DoubleSummaryDataPoint",
+ description: "// DoubleSummaryDataPoint is a single data point in a timeseries that describes the time-varying values of a Summary of double values.",
+ originFullName: "otlpmetrics.DoubleSummaryDataPoint",
+ fields: []baseField{
+ labelsField,
+ startTimeField,
+ timeField,
+ countField,
+ doubleSumField,
+ &sliceField{
+ fieldName: "QuantileValues",
+ originFieldName: "QuantileValues",
+ returnSlice: quantileValuesSlice,
+ },
+ },
+}
+
+var quantileValuesSlice = &sliceStruct{
+ structName: "ValueAtQuantileSlice",
+ element: quantileValues,
+}
+
+var quantileValues = &messageStruct{
+ structName: "ValueAtQuantile",
+ description: "// ValueAtQuantile is a quantile value within a Summary data point",
+ originFullName: "otlpmetrics.DoubleSummaryDataPoint_ValueAtQuantile",
+ fields: []baseField{
+ quantileField,
+ valueFloat64Field,
+ },
+}
+
 var intExemplarSlice = &sliceStruct{
  structName: "IntExemplarSlice",
  element: intExemplar,
@@ -404,6 +460,14 @@ var explicitBoundsField = &primitiveField{
  testVal: "[]float64{1, 2, 3}",
 }
 
+var quantileField = &primitiveField{
+ fieldName: "Quantile",
+ originFieldName: "Quantile",
+ returnType: "float64",
+ defaultVal: "float64(0.0)",
+ testVal: "float64(17.13)",
+}
+
 var isMonotonicField = &primitiveField{
  fieldName: "IsMonotonic",
  originFieldName: "IsMonotonic",