Add links to Prometheus and OpenMetrics summary

opentelemetry/proto/metrics/v1/metrics.proto

@@ -219,9 +219,11 @@ message DoubleHistogram {
 }
 
 // DoubleSummary metric data are used to convey quantile summaries,
-// a Prometheus and OpenMetrics data type. These data points cannot
-// always be merged in a meaningful way. While they can be useful in some
-// applications, histogram data points are recommended for new applications.
+// a Prometheus (see: https://prometheus.io/docs/concepts/metric_types/#summary)
+// and OpenMetrics (see: https://prometheus.io/docs/practices/histograms/#histograms-and-summaries)
+// data type. These data points cannot always be merged in a meaningful way.
+// While they can be useful in some applications, histogram data points are
+// recommended for new applications.
 message DoubleSummary {
   repeated DoubleSummaryDataPoint data_points = 1;
 }