[exporter/exporterhelper] Add DroppedItemsErr to report intentionally dropped items

[Krishnachaitanyakc]

Description

Exporters that intentionally discard items (e.g. a Prometheus exporter dropping non-monotonic DELTA sums) previously had no way to signal this to the exporterhelper observation layer. Dropped items were silently counted as successfully sent via otelcol_exporter_sent_metric_points, making internal telemetry misleading and difficult to use for diagnosing data loss.

This PR adds a lightweight mechanism for exporters to report intentionally dropped items:

• experr.DroppedItemsErr / NewDroppedItemsErr(count, reason): a non-failure sentinel that an exporter's push function can return to report how many items were intentionally dropped and why.
• Four new telemetry counters (otelcol_exporter_dropped_{spans,metric_points,log_records,profile_samples}): emitted by obsReportSender when a DroppedItemsErr is received.
• obsReportSender adjustments: dropped items are subtracted from the "sent" count, and DroppedItemsErr is not propagated as a pipeline error (the pipeline sees nil).

A follow-up to the prometheusexporter in opentelemetry-collector-contrib will be needed to actually return DroppedItemsErr for unsupported metric types (e.g. non-monotonic DELTA sums).

Resolves open-telemetry/opentelemetry-collector-contrib#48172

[mattclay]

@Krishnachaitanyakc Why did I get pinged on this? I'm not involved with this project.

[Krishnachaitanyakc]

@Krishnachaitanyakc Why did I get pinged on this? I'm not involved with this project.

my bad, i was tagging you on a different PR and got it mixed up, apologies

[Krishnachaitanyakc]

@bogdandrutu @dmitryax could you take a look at this change when you have a chance?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.13%. Comparing base (346b065) to head (0d73dcb).

Additional details and impacted files

@@           Coverage Diff            @@
##             main   open-telemetry/opentelemetry-collector#15028    +/-   ##
========================================
  Coverage   91.13%   91.13%            
========================================
  Files         701      701            
  Lines       45798    45902   +104     
========================================
+ Hits        41739    41834    +95     
- Misses       2845     2851     +6     
- Partials     1214     1217     +3     

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[codspeed-hq]

Merging this PR will improve performance by 27.7%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 2 improved benchmarks
✅ 5 untouched benchmarks
⏩ 76 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	BenchmarkPersistentQueue	181.7 µs	143.6 µs	+26.57%
⚡	BenchmarkMemoryQueueWaitForResult	61.4 µs	48.1 µs	+27.7%

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_{Comparing Krishnachaitanyakc:fix/issue-13643 (0d73dcb) with main (346b065)}

Footnotes

1. 76 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[dmitryax]

We need #15280 to land first