[exporterhelper] Add exporter_dropped_* reporting capability and metrcs

[bogdan-st]

Description

Add exporter_dropped_* metrics to track dropped items, allowing exporter devs to report back the number of dropped items.

Link to tracking issue

Fixes #13643

Documentation

Added instructions for exporter devs

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.14%. Comparing base (97fcd3d) to head (7d08f95).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13704      +/-   ##
==========================================
- Coverage   92.15%   92.14%   -0.01%     
==========================================
  Files         668      668              
  Lines       41517    41597      +80     
==========================================
+ Hits        38258    38329      +71     
- Misses       2221     2227       +6     
- Partials     1038     1041       +3     

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[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 14 days.

[ArthurSens]

Hey, anything I could do to help move this forward? Prometheus exporters drop data based on specification rules, would love to start using the new metric :)

[bogdan-st]

Hello @ArthurSens, yeah ill take a look at this today/tomorrow and update it, sorry for leaving it like this.

[ArthurSens]

FYI @mx-psi, this will be needed to stabilize Prometheus receiver. We currently drop metrics due to spec incompatibility and user can't observe this behavior

Wait, I got confused here lol
This is needed for Prometheus exporters, not the receiver if I'm not mistaken

[bogdan-st]

@ArthurSens do you have an issue for what you're describing? I think this issue might be also happening in the receiver side but ill have to investigate that. I can open a PR if I find something related to receiver dropped metrics.

[ArthurSens]

It's part of open-telemetry/opentelemetry-collector-contrib#39706, we need to have a metric explaining when metrics are dropped. For the Prometheus exporters for example, we currently drop Delta metrics because Prometheus doesn't support deltas

[bogdan-st]

Is there anything else I could do here to get this merged?