update links to openmetrics to reference the v1.0.0 release (#612)

Author: dashpole

opentelemetry/proto/metrics/v1/metrics.proto

@@ -452,7 +452,7 @@ message HistogramDataPoint {
   // 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
+  // see: https://github.com/prometheus/OpenMetrics/blob/v1.0.0/specification/OpenMetrics.md#histogram
   optional double sum = 5;
 
   // bucket_counts is an optional field contains the count values of histogram
@@ -531,7 +531,7 @@ message ExponentialHistogramDataPoint {
   // 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
+  // see: https://github.com/prometheus/OpenMetrics/blob/v1.0.0/specification/OpenMetrics.md#histogram
   optional double sum = 5;
 
   // scale describes the resolution of the histogram.  Boundaries are
@@ -645,7 +645,7 @@ message SummaryDataPoint {
   // 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#summary
+  // see: https://github.com/prometheus/OpenMetrics/blob/v1.0.0/specification/OpenMetrics.md#summary
   double sum = 5;
 
   // Represents the value at a given quantile of a distribution.