ostream metrics example

[esigo]

related to #1092 (issue)

Changes

For testing end-end metrics
Adds ostream metrics example and fixes Export().

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 #1312 (454a2c9) into main (d72dcb6) will decrease coverage by 0.35%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1312      +/-   ##
==========================================
- Coverage   91.70%   91.36%   -0.34%     
==========================================
  Files         217      217              
  Lines        7842     7842              
==========================================
- Hits         7191     7164      -27     
- Misses        651      678      +27     

Impacted Files	Coverage Δ
...nclude/opentelemetry/sdk/metrics/metric_exporter.h	50.00% <ø> (ø)
...entelemetry/sdk/metrics/view/instrument_selector.h	100.00% <ø> (ø)
...metrics/export/periodic_exporting_metric_reader.cc	43.19% <0.00%> (-34.09%)	⬇️
...t/metrics/periodic_exporting_metric_reader_test.cc	64.52% <0.00%> (-29.03%)	⬇️
sdk/src/metrics/metric_reader.cc	55.56% <0.00%> (-8.33%)	⬇️

[lalitb]

This may need further changes, based on the review for #1299, as it adds Resources and Instrumentation Info to Metric Data to be exported.

[lalitb]

#1299 is merged. you can modify the PR to use ResourceMetrics instead of MetricData.

[lalitb]

@esigo - Just want to check if you want this to be reviewed now, as I was thinking to review it once it incorporates changes for ResourceMetrics as mentioned above :)

[esigo]

Hi @lalitb, I'm rewriting the unit tests, should be done by tomorrow. I'll ping you when I'm done.
Thanks

[lalitb]

Thanks, @esigo. Just wanted to ensure that I am not delaying the review comments in case you are expecting it to be reviewed :)

[esigo]

@lalitb this should be ready for review, thanks :)

[lalitb]

nit - should we add small metrics instrumentation under (example/common) similar to how we do for traces and logs?

[esigo]

thanks, added.

[lalitb]

nit - need to check, but I think this can be directly obtained by calling time_stamp()?

[esigo]

I try to print time in human readable format here. I didn't find any function similar.

[lalitb]

Not sure if something wrong with GitHub. I pasted a comment here, it was displayed multiple times and then when I removed one occurrence all of them removed :) But it was not that important, was thinking something to avoid couple of lines of code (so can be ignored):

std::string timeToString(opentelemetry::common::SystemTimestamp time_stamp)
{
  std::time_t epoch_time = std::chrono::system_clock::to_time_t(time_stamp());
  return std::ctime(&epoch_time);
}

[esigo]

thanks, added your version.

[lalitb]

instrumentation library shouldn't include any sdk component. Can we remove these includes ?

[esigo]

thanks, removed.

[lalitb]

Thanks. LGTM.