[EXPORTER] Prometheus: Remove explicit timestamps from metric points

[punya]

Fixes #2316

Changes

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #2324 (e9f556b) into main (ca67af0) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2324      +/-   ##
==========================================
+ Coverage   87.45%   87.47%   +0.02%     
==========================================
  Files         199      199              
  Lines        5997     5997              
==========================================
+ Hits         5244     5245       +1     
+ Misses        753      752       -1     

[marcalff]

@punya Thanks for the fix.

The failing CI is due to:

/home/runner/work/opentelemetry-cpp/opentelemetry-cpp/exporters/prometheus/src/exporter_utils.cc:47:12: error: unused variable 'time' [-Werror,-Wunused-variable]
      auto time          = metric_data.end_ts.time_since_epoch();

[lalitb]

LGTM once the CI is fixed.

[lalitb]

Format needs to be fixed, either by running cmake-format, or adding the patch below:

--- a/exporters/prometheus/test/exporter_utils_test.cc
+++ b/exporters/prometheus/test/exporter_utils_test.cc
@@ -43,7 +43,8 @@ void assert_basic(prometheus_client::MetricFamily &metric,
   ASSERT_EQ(metric.type, type);                      // type translated
 
   // Prometheus metric data points should not have explicit timestamps
-  for (const prometheus::ClientMetric &cm : metric.metric) {
+  for (const prometheus::ClientMetric &cm : metric.metric)
+  {
     ASSERT_EQ(cm.timestamp_ms, 0);
   }

[marcalff]

Thanks for the fix.