exporter/otlploghttp: guard observ metrics with Enabled checks

[NesterovYehor]

This PR updates the OTLP HTTP log exporter observability instrumentation to check instrument Enabled(ctx) before building options/attributes and recording measurements.

Behavior is unchanged when metrics are enabled; this reduces overhead when exporter self-metrics are disabled/no-op.

Part of the work tracked in #7800.

[itssaharsh]

Hey, I wante to ask you check this but what if something changes in between time start (sorry i was wrong i was using your code as refrence to understand what the changes were )
if i.operationDuration.Enabled(ctx) {
extOp.start = time.Now()
}
duration := time.Since(e.start).Seconds()

[codecov]

Codecov Report

❌ Patch coverage is 82.14286% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.7%. Comparing base (f740ea8) to head (9458d23).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
...log/otlploghttp/internal/observ/instrumentation.go	82.1%	1 Missing and 4 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #7813     +/-   ##
=======================================
- Coverage   86.1%   81.7%   -4.5%     
=======================================
  Files        302     304      +2     
  Lines      22046   23247   +1201     
=======================================
+ Hits       18991   18998      +7     
- Misses      2675    3859   +1184     
- Partials     380     390     +10     

Files with missing lines	Coverage Δ
...log/otlploghttp/internal/observ/instrumentation.go	93.6% <82.1%> (-6.4%)	⬇️

... and 10 files with indirect coverage changes

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[dmathieu]

Could you add tests?

[NesterovYehor]

Hi. I can add tests.

Do you expect separate cases (drop inflight/exported/duration, plus “all dropped”), or is one view-based test enough to show that when an instrument is disabled we skip it and ExportLogs().End() still records the remaining metrics?

[dmathieu]

I think one test with subtests for each metric would be good.

[dmathieu]

Don't trust what the LLM says. Your test needs to fail before your changes, and pass after.
Here, it passes both before and after.

[dashpole]

How would this be tested? Enabled is a performance improvement to avoid computing attributes when we know in advance the metric will be dropped. Using enabled or not shouldn't be a behavioral change, right?

[dashpole]

Might be able to demonstrate it with a benchmark when a no-op meterprovider is used?

[NesterovYehor]

I added a unit test because another reviewer asked for tests. it basically just verifies we don’t call Add/Record when an instrument reports Enabled(ctx)==false.

If you’d prefer, I can drop the unit test and add a benchmark instead since this is mainly a perf improvement. What would you rather have?

[dashpole]

Nice. I didn't think of that, and that works for me.

[NesterovYehor]

Awesome, thanks! I’ll fix the remaining lint issues and update the PR.