Add synchronous gauge

[uuzay]

Fixes #2279

Changes

Added synchronous gauge implementation according to the specification.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 87.60%. Comparing base (497eaf4) to head (bd8cb23).
Report is 117 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3029      +/-   ##
==========================================
+ Coverage   87.12%   87.60%   +0.49%     
==========================================
  Files         200      190      -10     
  Lines        6109     5870     -239     
==========================================
- Hits         5322     5142     -180     
+ Misses        787      728      -59     

Files	Coverage Δ
api/include/opentelemetry/metrics/meter.h	100.00% <ø> (ø)
api/include/opentelemetry/metrics/noop.h	41.18% <ø> (ø)
...i/include/opentelemetry/metrics/sync_instruments.h	100.00% <ø> (ø)
...etry/sdk/metrics/aggregation/default_aggregation.h	70.13% <ø> (ø)
sdk/include/opentelemetry/sdk/metrics/meter.h	57.15% <ø> (ø)
...clude/opentelemetry/sdk/metrics/sync_instruments.h	100.00% <ø> (ø)
sdk/src/metrics/meter.cc	62.36% <ø> (+0.91%)	⬆️
sdk/src/metrics/sync_instruments.cc	65.84% <ø> (ø)

... and 118 files with indirect coverage changes

[marcalff]

Thanks for the feature.

This is not a full review yet,
only some preliminary comments.

Please fix:

• declarations of new virtual methods using ABI version 2
• build break issues found in CI

More extensive review to follow.

[marcalff]

Thanks for the feature.

The code structure looks good to me overall.

I defer to @lalitb to check the logic around aggregation temporality in particular,
as I am not too familiar with this area.

[marcalff]

It will be simpler to just declare this enum value all the time, and not conditionally.

The sdk headers can change, and having kGauge declared even in ABI v1 is not disruptive.

It will just be dead code in ABI v1.

Please adjust the switch/cases accordingly, to avoid the ifdef

[lalitb]

@uuzay Thanks for the PR. Had a quick glance to the code, this looks similar to #2341 (correct me if not). And I believe this won't work for delta temporality. To validate, can we update the PR to test for it?

[lalitb]

Update the test here to include the delta temporality.

[punya]

@uuzay Thanks for the PR. Had a quick glance to the code, this looks similar to #2341 (correct me if not). And I believe this won't work for delta temporality. To validate, can we update the PR to test for it?

If I understand the relevant section of the spec correctly, Gauges (unlike UpDownCounters and Histograms) don't support a choice of temporality.

[uuzay]

I was thinking the same @punya.

It also mentions:

Gauges do not provide an aggregation semantic, instead “last sample value” is used when performing operations like temporal alignment or adjusting resolution. (source)

if that's related.

@lalitb can you please clarify if delta temporality is actually needed?

[lalitb]

@lalitb can you please clarify if delta temporality is actually needed?

Sorry i was not clear enough. I meant testing for temporality with last value aggregation. The current test is only doing for cumulative.

[lalitb]

@uuzay - Do you think we can modify the PR to restrict configuring the delta temporality for Last Value as of now - Just thrown an error at the startup if this is enabled through View or otherwise. With that, we should be good to go through this PR.

Thanks for working on this, and sorry about the long wait on this.